I did that for my Meroko costume.
If u've already got the top hat, then it's pretty easy. Just use that as your guideline.
Trace around your top hat on a sheet of paper to create your own top hat slipcover pattern.
trace the top circle of the hat
The base of the hat - and the hole for your head to go through
and then measure the tall part going around.
If you cut these pieces out of paper first, u can see if it's going to fit or not when u cut them again, out of the fabric
Then u can sew them together n slip them right over the hat - or, do what i fif and sew them or glue them directly onto the hat itself.
it really sounds more complicated then it really is