I have always tucked while standing up. I feel like it'd be harder for me laying down, but haven't experimented much. I just push the 2 bits up with one hand, pull the other part back freeing your first hand at the same time) with the other hand and pull up my panties with the hand that is now free. Everything just kinda stays in place for me.
I have not worn panties that were small on me, generally I just wear the bikini style cotton panties that come a pack. This has always worked super well for me and I'm always comfy. I tuck pretty much 24/7 including when swimming, skateboarding and various other physical activities with little to no issues. Every once in a while I find myself in an emergency situation I need to re-tuck, but since I always stand up while tucking its generally (as long as clothing permits) easy to pull off without people noticing.
Now that I have been on HRT somewhere between 5 and 6 months, it has become easier and more reliable than it was pre-HRT. I think this is mainly due to stuff getting smaller and remaining inactive throughout the day.