I start the plan on Maps with departure-destination. Enable the 100LL price layer, look for places I might want to stop - and I'll use the along-track offset here as well to split the flight into 2 or 3 evenly spaced sections as well - and then I'll replace them with my actual desired stops.
Once I have the stops, I'll split it up. This could be easier, but it's pretty trivial to just delete everything after the departure and first stop, add any routing, procedures, altitude, etc specific to that leg and Send To -> Flights, and then go back to Maps and hit your recent flight plans to get back to the full trip route, delete the departure and anything after the second stop, rinse and repeat until you have a Flight for each leg.
From there, it's pretty easy. Hit the Flight, File, repeat as necessary. Then you've got the Flights that you can send to Plates and Maps before you fly each leg.
You can also put the full route in its own Flight so that you can brief the entire route and retrieve it that way to split it up.