I've seen so many pictures in the gallery of a wedding shower dress cake that's usually rectangular... how would I do the draping on it?

I hope that made sense!

If it's the flat sheet cake type it's done by pleating. I've not done one yet though.

roll your fondant larger than "dress" pleat to gather & make drape effect.

What shape would you cut the bottom of the dress into before pleating?

It can be easly cut afterward with scissors, or given a handerchief or scallop edge before placing on cake. Make a small "test" dress to determine the shape.

I actually did mine in individual sections because smaller pieces of fondant are easier to handle. I just tucked the edge under to give it the pleated effect. I cut the bottom after it was on the cake.

I suggest using the back of the pan you will use or used for the cake that way you will get the almost perfect fit.
Also to do the pleats I use Chop-sticks to help get them even and it's easy to lift.

Hope this makes sense.

Good Luck.