Disclaimer: I mostly do my route planning from google maps, as this makes it real easy to research as you go...
As noted above, first thing is to decide where you want to go (who knew!?!?!) looking for a set of main controls (defacto, rest/feed points which generally means one for every 50-80km) you want to use, then pin the route out to use your preferred roads.
If your DIY Perm has an Advisory Route (where the distance is defined by the 'shortest distance between controls') , you may need to add additional controls to make up the distance (devising a route that goes where you want with a mininmum of controls is all part of the fun!).
if your DIY Perm has a Mandatory Route (defined by the GPS track submitted with your entry), additional controls will not be required, as the route/distance is detailed in the route GPS track.
If you're planning to VALIDATE using a GPS track you need not stop at any of the controls, however, its still good practice to list the controls/major waypoints on your DIY Perm entry. Besides being an AUK requirement, this information is useful for documentation purposes.
Note for South East region riders: when repeating a route previously registered with me, either by you or another rider, use the 'Advisory DIY' entry form and enter the previously registered *DIYSE Brevet Number* as your start and finish control, and I'll pick up the relevant routing info from my records.
Edited to tidy up some POP (Points of Pedantry.