Fenders
The fenders can be made from a template, but if you change the wheel size, you'll probably want to adjust the fenders to match the wheels you've made. I tend to make the fenders ¾” thick, or maybe a little more, but since I also make them two at a time from the same board, I use 8/4 lumber at a minimum.
I generally do not use the template. I will mark the positions of the wheels and body on the fender blank, and then sketch the fender shape using curvy things to make them look good. But I made a template, and if I need to make a large batch of identical cars, I'll use that. I would suggest not using the template until you've made a few cars, and tweaked them to match your own style.
The next step is cutting them out. Then smooth the tops and bottoms with a rasp and 60 grit sandpaper.
And then cut the two fenders apart using a bandsaw or a hand-saw.
Next, mark which sides are going to get glued to the body (I tend to use what was the outside of the board, as commercial lumber is smooth enough to glue together as long as there are no visible flaws), and clamp one fender in place. Drill a pair of ⅛” (3mm) alignment holes about ¼ inch (6mm) deep into the fender material through the body. Bamboo skewers (sold for grilling meat) will fit nicely into these holes, and will make gluing things up easier. Then unclamp that first fender and align it with a couple pieces of skewer, and clamp the other fender onto the opposite side of the body so it's lined up about the same, and drill holes into the second fender through the same holes you made in the body with the first fender.
Then look at the body, the wheels, and the fenders, and decide how much you're going to taper them. I tend to leave the fenders a little wider than the wheels. In any case, you want to make the two fenders as symmetric as possible, and make a tapering cut from the front end to somewhere in the middle of the fender.
Once those cuts are done, finish smoothing the fenders and finish them, leaving the sides that will be glued to the body unfinished (so glue will stick). If you want to add lights, racing numbers, or other decoration to the fenders, now's the time for it. Paint or not, I try to put 3-5 coats of tung oil on, giving each thin coat a day to cure, so this is a slow part of the process.
And there's your fenders!
Discuss... Or contact me in the fediverse @davepolaschek@writing.exchange