Thread: Creating a custom map?

I hate asking this, but there are so many posts with different info... so if someone can point me in the right direction, I'd be grateful.
I have an old Nuvi 265W. I want to upgrade the map. The unlocked 2015.40 NA Lower 49 states, is just a bit too large
(240 MB). I've deleted everything I can, & the gps doesn't seem to be recognizing the sd card.
So, I'd like to create a map of the mainland, without all the Islands (Hawaii, Puerto Rico, etc.), which I imagine will do the trick for me. For now the Northeast would work, but that version is on a filehost that doesn't work for me (upload.ws).
What tools do I need? I see Mapsource, Basecamp, unlocked, locked... just not sure the most efficient, effective
way to do it. Or if there's a step-by-step tutorial?
Thanks for any help... appreciated!