My opinion only - others exist

What I'd suggest is get an A4 pad and a couple of different colour biros.
On each page (in colour 1) write the name of a THING that the system has to deal with
(e.g. CLIENT, INVOICE, ORDER, etc.)

For each thing make a list of the PROPERTIES of the thing you need to work with.

The under that list the ACTIONS or operations that can happen to that THING

Then at the bottom page list:-
What things are related to this thing
Do I need to store information about this thing

From this build your class library and database (if approrpiate)
Don't do the UI until last because it is the most fun part...