Hi Sabrina! I'm sitting here having been tucked all day with a VS cotton thong. This tucking thing does take practice and it's good to start with all the marbles in your court, so to speak. Here's what I did.
Take your tightest pair of jeans, and your tightest panties. Pull the panties on almost all the way, put the "boys" up into their cavities (they might bulge a little at first), and while holding them up with one hand, reach around from the back and pull the long guy through. By pulling on him, the two "boys" should stay in place. Then, pull up your panties until you can hold him through the panties. Pull the panties the rest of the way up, tightening by pulling the thong part of the pantie to the back. This keeps everything tight while you put on your tight jeans. This should give the jeans an ok front and a neat tuck.
When you have to use the "little girls room" (or whichever room!), just reset the tuck. If the tuck has come undone, don't fret, just reset it. I did this every day for a week and sometimes the tuck came undone, but by the end of the week the little bulges were decreasing (from the boys) and everything was learning where to stay.
It just takes a little time for everything to get used to where it belongs, and then you are set. When I transform en femme, and I want it really tight with no jeans (wearing leggins or such), I do use a gaff as it is a bit stiffer, but now that everything is trained, it really is remarkably flat in front, and I'm not all that small
Have fun!
