I think many of us have a particular motivation that gets us into little space. I like watching cartoons (some cartoons at any rate) and playing games, and if I'm all dressed up in a diaper and footie PJs hugging a plushie while watching My Little Pony, I start to feel pretty little. I'm not thinking to myself "time to feel little" though, I'm thinking to myself that I want to be dressed little and I want to watch this show, and I want a plushie to hold because I want those things sometimes even when I start out feeling like a normal adult.
If you really don't have any particular thing you want to try, but you want to feel little and are struggling to find a way to get there, I'd suggest something random. Grab your nearest boardgame and 6-sided die, and make a list of a few things: watching a cartoon, playing with some toys, coloring or drawing a picture, I dunno, there are plenty of suggestions and I'm sure people here in the thread can come up with more. Just list out 6 and roll and see which one you get. Put on a diaper, and then try whatever activity you got, see if it works.