I don't understand how people are saying they use guides over two layers of the base colour gel polish. Firstly guides/stickers will not stick to the inhibition layer of the gel polish (and you won't want to cleanse untopcoated colour). Secondly the colour bleeds under the stickers, I have tried will all sorts of stickers. Thirdly if you flash cure and remove the sticker it will create a ridge.
I wonder if some people are saying what they think they would do to achieve this effect as opposed to what they have tried.
I have done this a few times and honestly the easiest and quickest way is to to it freehand. If you can do a french manicure, you will be able to do this. It is easier than it looks. Have a practice on some tips.