I'm only a Newbie, just started my first project, but after having made my first couple of routing templates I would give this advice:
STAY AWAY FROM MDF!!
I tried to make a routing template for a neck pocket, and found the stuff totally unworkable. Try to shape it with surform and/or sandpaper, and it just turns into mush. The best surface I could achieve was fuzzy and unreliable - the bearing of the guided router cutter just sank into it. Lucky I was only practicing on scrap!
Later I tried it with a nice sheet of ply-wood, with much more pleasing results - easy to rout and sand, with a nice finish. Now I've got a nice neck pocket template - it's a perfect fit for the neck heel and the surface that will contact the router cutter bearing is nice and solid. I'm v pleased with it as a first attempt, I just wish I hadn't waster an evening messing about with MDF - but as this thread is all about, mistakes are part of the learning process.
It's true - mistakes are another way of saying "I am wiser today than I was yesterday!"