It's all about being innocent and natural for me. (Or was, this kind of play happened mostly a long time ago, around puberty.) The ways I played most often were:
- Feeling like a 2 year old, still in diapers, who's knows he's going to pee or poo, and stops and feels all the sensations in his diaper as it happens, and probably likes the feelings quite a bit, and then just carries on playing while messy or wet, it's all part of his normal day.
- Being a 2 or 3 year old or so who's officially trained but still has accidents pretty often, and isn't uncomfortable or self-conscious about them yet, he just watches kind of curiously as his pants fill or (slowly or quickly) get warm and soaking wet down the front.
- Being in the middle of toilet training, getting the urge to go, and hurrying toward the bathroom, but doing it all in my pants on the way there (and being young enough that this expected, and seen as funny, and cleaned up kindly, and treated as a 'good try').
- Being 3 or 4, and half waking, with an urgent need to pee, and just doing it disorientedly right there in my pants or bed.
- Being a toddler away from home, and afraid to use an unfamilar toilet, and just holding on until I can't any more and it happens in my pants (a scenario right out of my parents' copy of Dr. Spock).
- Occasionally, being a smaller baby who just wets at frequent intervals as the urge comes, and feels it but without much interest and with no control.
Finally (and messily), the very first poop scenario I ever tried, out of sheer curiosity about what it was like to be a baby, and to have to clean one up. Being 12 or 13 months old, learning to walk, curious, adventurous, unsteady on my feet. I have poop in my pants, and toddle about and fall down on my bum, over and over again. (Inspired by a year-old relative I saw do exactly this, and then get very messily changed.)