Again this isnt my idea, but I have done it many times in the last few years. It first poped up on a fetish site and then hit it big on a diaper related site, I am just sharing the information.
Their are lots of ways to do it. I have more recently found it best to use Suppositorys, 2 or 3 and then as many bananna's as you can till it hurts, then I go in with a Poise medium pad. Last time I did this, which was 2 nights ago, I cut slits in the poise pad and then rolled it up with a enema tube inside it. When I inserted it I attached the enema tube to the bag and let the water pour in and make that pad bulge up, pulled it out after a few minutes and diapered up.
You can also just go with as many banannas as you can and then the pad, it will come out eventually. The longer the pad is in you the more it absorbs and thats the key, you want it to have to work its way out
Tampons also work like someone said, I have done it with 3 supers, inserting all 3 tubes at the same time and then pushing the plunger thingys, the sweel up real quick and big.
Oh I also usually use a couple fingers at first to loosen up and get the real ripe banannas.