This one is pretty easy - you don't do anything! You just say "Thanks, but I'm not interested in doing [X] right now."
It's up to you whether you feel like explaining. You can give a reason, but you don't have to. You may have heard the phrase "'No' is a complete sentence." That means you can just say no without further explanation.
A good partner will accept your 'no' respectfully. You communicating what you're comfortable with is a gift they should be happy to receive! Some people might not agree to a 'no' so easily, and try to get you to say yes. Learning to stand up for yourself is an important part of any relationship, whether or not sex is involved. We've got lots of info on that here: Be Your Own Superhero: Learning How and When to Stand Up for Ourselves