Here's How:

1. Clarify the Issue for Yourself
   Sex is complicated, your feelings may have as much to do with your own baggage and history as with your partner. If something is on your mind, sit with it for a while and clarify what doesn’t feel right. Some people find that writing a helpful way to do this; others will talk with close friends. The point of this is not to start complaining to others or placing blame. The exercise is all about you and how you feel.
2. Try to Write it Down
   Don’t worry about floral language or grammar. Writing down what you want to talk about is a great step to clarify your issues for yourself and practice the way you might communicate it to your partner. Some people actually write their partner a letter, and end up giving it to them at a later point. Letter writing can be a powerful way to communicate your thoughts and feelings, and if done along with talking it can increase intimacy in a relationship in surprising ways.
3. Practice the Talk
   This doesn't work for everyone, but if you’re nervous about talking it can help to do some practice talking. If you have a good friend you can do this with, great. If not, going through it on your own can help. Before every major “talk” I’ve ever had to do, I sit down in front of my computer screen and practice. If my monitor could talk, it would probably tell me to get a life, but it’s a great tool for me (and I’m pretty sure my monitor is too old a model to be talking).
4. Consider Your Timing
   One of the most important considerations is when to talk. This will depend a lot on what you are talking about. If you want to raise the idea of trying something new in bed, then raising it just before you’re about to be intimate with your partner is probably not a good time. Ditto for raising your dissatisfaction with the frequency of your sex life minutes before your kids are due home (or your in-laws are coming over).
5. Choose Your Location
   As with timing, location can make a difference. Bringing up sexual dissatisfactions in bed can be a bad move as it may create an association of negativity in your bed. Both you and your partner may also feel more vulnerable in bed than you would having the conversation fully clothed, out for a late-night walk.
6. Allow Time for Processing
   Remember that your partner may be surprised by what you are saying. Give both of you the time and space to respond honestly without having to feel rushed or pressured. You may not be able to completely resolve the issue, or even talk about all the aspects of the issue in one sitting. Think about sexual communication as an ongoing process, not a one-shot deal.
7. Be Generous
   No matter how hard it is for you to bring up your sexual concerns, if you are the only one raising them, consider that it may be even harder for your partner. If you can, try to be generous with your partner and try not to place the blame too heavily on either of you. There are two of you in the relationship, and ultimately both of you need to take responsibility for what is happening.
8. Check In Afterwards
   Sometimes we can make ourselves so anxious about bringing something up with a partner, and then it goes not quite as bad as we thought, and we’re relieved, so we want to just move on. Give yourself permission to bring the topic up again. Don’t do it in a nagging way, but make it clear that your partner that you care about how they feel and you want to check in with them about how the conversation went.

Tips:

1. Keep in mind that every situation is different. These are general tips and your situation may call for many additional considerations.
2. Remember that your imagination can be your worst enemy when it comes to taking risks like this. The reality is that the response is almost never as bad as you think it will be, and talking openly about your sexual feelings, desires, likes and dislikes, can not only improve your sex life with your partner, it can improve other aspects of the relationship.

Whenever we bring up a new sexual conversation with a partner, we are exposing a part of ourselves. Even with long-term partners this can feel so risky that we avoid it altogether. But keeping lines of communication open is essential to keeping a long-term sexual relationship alive. One way to make these conversations feel safer is by establishing ground rules for sexual communication.

Here is a list of sexual communication ground rules. If you’re looking for a way to start a new (and uncomfortable) conversation one way to start is by talking with your partner about this list, and developing your own set of rules. You can pick and choose from these ideas, or use them as a launch pad to develop whatever rules feel right for everyone involved.

Time to Check In and Check Out

If you’re sitting down to talk when you’re exhausted or angry about something else that happened that day, you’ll bring those things into the conversation with you. One way to deal with this inevitability is to “check in” at the beginning of the conversation; spend a minute talking about your day and how you’re feeling so you know what you’re each coming to the conversation with. You can also take a minute at the end of the conversation to say how you felt it went (but don’t use that as a way to keep the conversation going after you’ve agree to end it).

Define Safety for Yourselves and to Each Other

No one can be expected to talk honestly and openly unless they feel safe. Make a list for yourself about what you need to feel safe and then share that list with your partner. If you can agree on things from your lists those can become part of the ground rules.

Respecting Differences in Sexual Interests

If the conversation you want to have is about sharing sexual fantasies or things you might want to do together, everyone needs to agree not to belittle, shame, or otherwise laugh at their partner’s sexual interests. This doesn’t mean you have to like them or act them out. But if your partner takes the risk of exposing a sexual desire, you need to consider that kind of sharing a compliment and be respectful, even if you want nothing to do with the actual proposal.

Listen Without Interrupting

This is one the hardest ground rules to follow, but also one of the most important ones. If you’re interrupting and talking over each other, neither of you will ever feel heard. Agree to not interrupt and know that you’ll both slip up, but when you do you’ll apologize and try harder to listen and wait until your partner is finished before responding.

Bring a Sense of Goodwill to the Conversation

Hopefully it’s true of your relationship that neither you nor your partner are trying to intentionally hurt the other or be mean to the other. If you believe this is true, try to remember it even when you’re getting into it with each other. If you find yourself attacking (either in what you’re saying or how you are saying it) remember this ground rule and back off a bit.

Make 'I' Statements

A basic rule of good communication is to avoid telling the other person how they feel or what they think (in reality, you have no idea). What you can do is talk about yourself and your own feelings. And a simple way to do this is try to start sentences with “I”, as in “I feel like this when you do that”.

Respect Differences in Values and Feelings

Relationships require a lot of compromising, but it doesn’t mean we don’t get to have our own thoughts, values and feelings. Show each other enough respect to all for differences without forcing either person to concede their beliefs. In the end, everyone has to compromise, but in the beginning everyone should be allowed to be heard.

Be Specific, Ask Questions

Try to be as specific as you can be and avoid making sweeping general statements (e.g. “you always do this” “I can never do that”). If you find your partner making vague or general statements ask for clarification and ask questions to try and help both of you get a clearer idea of what specifically is the problem.

Anyone Can Call Time-Out at Any Time

For a conversation to feel safe, everyone needs to feel like they can leave at any point. This doesn’t make it OK to storm out while your partner is in the middle of a sentence, but agree that either of you, at any point, can ask for a time out, or to end the conversation, as long as you also agree to pick it up at a later point.

Agree on Confidentiality

Everyone needs to feel like what they say in a private conversation stays private. You’ll have to agree on the limits of that (is it OK to talk to a best friend? A brother or sister?) but whatever restrictions you put on it, you have to respect those and not break confidentiality.