I have been making my own specialized glass, and I start with making several lengths and varieties, such as the piece depicted here.
Then I cut those lengths into smaller pieces and design them on a base color. I don’t JUST use my specialized glass, but try to mix it up with other pieces, working out color and composition all the way along. The pieces are attached to the base glass with the tiniest speck of glue, just so they don’t shift as I put them in the kiln.
Piece of cake, right?