I usually do it at night, though that's only because I don't want anyone in my house to know I'm using them to mess myself (I use them to fill my diapers)
Usually I try to eat a lot on the day of and a bit before, and usually best results are when I have that slight urge where I could push out a load if I wanted, but it isn't cramping or that strong.
As for which suppo, it depends on what effect your body would give you, and what you're looking for. I've find the bisacodyl make me want to stop fighting and let it flow out, while the glycerin makes me want to push it out after fighting it and then keep leaking out after I start, not letting me stop. Warning though, the bisacodyl makes you feel like you need to keep going after you're cleaned out though, so depends on how much time you have for it. glycerin, depending on how many you take and your body could take maybe an hour at best from start to complete finish, while my experience with two bisacodyl made me feel it's effects for two hours just in my diaper., with it lasting about half that time AFTER the movement
Try experimenting, also, if you want something strong, mix those suppos with some chilled marshmallows, that'll probably make it fire out of you