I think this might be a silly question, but can I use the bismark tip to pipe filling into any old cupcake, or is there something special involved? I mean, where does the part of the cupcake that used to be where the filling is go to? There is no hole in the cupcake, so how does the filling fit???

Yes you can use the bismark tip. The cake will move over some when you fill, if you put too much filling in it will crack so do a few for practice to make sure you have enough filling but not too much.

Speaking of this bismark tip. Do you use a coupler? I can't see to figure it out. When I don't use one the tip falls through the bag. Am I doing something wrong?

I use the coupler with mine. the base of it that goes onto the bag is the same size as other tips and it fits into a coupler well.

As for filling I usually poke my tip into it and do a quick 360 degree twist to kinda squish the cake out of the way then do another 360 while I am squeezing to fill in the cupcake. It definitely take a little practice to figure out how much to squeeze. I still break some open when I am not paying attention to what i'm doing.

Thanks to all for your answers - I am going to give it a shot!!

I fill my cupcakes in three places rather than one big squeeze in the middle. This way, it's distributed better and there's less chance of a blowout.