The 'how' of your question is the easy part...simply slide back gently onto your boyfriend's penis, using your hands to guide him inside you, then proceed to roll your pelvis backward and forward. Some women like to go in for a more vigorous bopping up and down action, but this simply comes down to personal choice (though you should know that bopping requires much more effort than rolling and can be uncomfortable for the woman if the angle is wrong as well as painful for her lover). Make sure your clitoris is stimulated by your actions and that your rocking is synchronised with your boyfriend's thrusts - it might take a while to get the movement right, but you'll get there with practice.
The other part of your question (the one relating to body image) is far trickier to respond to because where matters of self-esteem are concerned, there's always so much history to review. Your self-consciousness might stem from much earlier in life, for instance, perhaps from a time when a parent made you feel physically inadequate.
Or it might be a more recent occurrence resulting from, say, unexpected weight-gain or breasts that have grown, in your view, either too big or not big enough. Or it could originate from a rude comment made by a former lover. Regardless of the origin of this psychological block, however, there's only one way to reverse your low self-esteem: be kind to yourself. I know what you're thinking...easy to say, not so easy to do. But please try.
Don't demand physical perfection from yourself. Learn to love your individuality. And one final tip: allow yourself to trust your lover. If your boyfriend says you're gorgeous, and if your 'defects' remain invisible to his eye, then try to believe that you truly are sexy from every angle, and don't let self-doubt wreck your moments of intimacy. Let's face it, sex is only fun when you abandon yourself entirely to the act of love-making, without any thoughts for appearance or angle. So come on, stop worrying about your wobbly bits and go for it - hop on top!