In addition to Stephen’s good advice, you can also get a deep slump by using a drop ring and cutting off the “hat brim”. That is how this piece was done:
This can be done slumping into the air (traditional drop ring) or by cutting a custom ring out of fiber board to fit the mold of you choice. After slumping, the rim is cut off and the edge of the piece polished.
Lastly, many time you can slump directly into the deep mold twice. The first slump will almost certainly be lopsided – but the piece can then be repositioned for a second slump.
Helios Kiln Glass Studio