I wear a tucking gaff under tight pants every day. I'm above-average size and have had the sack skin falling out issue too. Generally with tucking you want your balls pushed up into those little pockets in your upper pubic area, where they were before they descended. It'll get easier over time as your balls shrink if you're on HRT. Mine just chill out there on their own a lot of the time now after a little over a year on HRT.
This is how I do it: First, put on the gaff loosely and position the front lower than you think it should be. This is so you can hike up the sides and the back to keep it tight later. It also helps keep the sack skin contained since the gaff will be positioned to be wider further down. After that, grab the head of your penis from behind, arm behind your back and reaching between your legs, reaching into the gaff, and pull your penis back between your legs, so that the head is almost to your asshole. Don't pull your penis too too far back or it'll be painful later. Pull up the gaff, tight so the penis stays in place, with sides and back going pretty high up to keep it tight. The gaff needs to be tight! The gaff should keep your balls up in those pockets and your penis tucked under without it slipping forward.
I use BBLAIR tucking gaffs in XS size with my 36 inch hips and 29 inch waist. (For me at least, I buy bottoms based more on hip measurement rather than waist, since my hips are a lot narrower than most cis women's, or at least typical women's pants.)
Thank you so much for the writeup! I'll keep this in my pocket until the long, long wait for my vagina is finally over.