okay well i'm not a makeup artist but i'll tell you what i know
basically, to make your lips look fuller you should definitely have lip liner because sometimes part of our lips can sort of blend into our face. if you don't want them to look too lipsticky then get a colour close to your own shade but don't get it too dark because dark colours make everything look smaller (same goes for eyeshadow)
basically what you want to do is line your lips at the very edge but don't let it get on your actual skin because that's fucking ugly. then use your finger to lightly smooth the line out so it doesn't look so harsh.
then you need lipstick. if you really want you could use the lip liner to shade in your lips but it's kind of a waste of lip liner. don't get an ugly shade of lipstick either, especially if you want it to look natural. basically what you need to do is apply the lipstick to your mouth sort of like this
O O
..O
the top two being your upper lip, the bottom is the bottom. then press your lips together like usual, blot on a kleenex and then put some powder on top so it sets. the colour should only be in the centre of your lips at this point. then apply the lipstick to all of your lips (it's best with a lip brush) and then blot again. and make sure the lipstick covers all your lips so when you open your mouth it doesn't have that ugly gap in the corner.
the part that's optional is putting lip gloss on the bottom lip or a semi-shimmery light powder .
i think that's it.
