This step is important, you need to figure out how your rack will look like after bending. I recommend to use rope it order to prototype shape of future rack.
My bike wheel is 28″ and I’ve decided that I want to have rack about 40 cm length… now I would use 35cm length of rack.
I would suggest bend slowly always checking with your bike. Rack should mount a bit like a spring
After bending most likely you’ll need to cute a bit of tube like I did. I’ve cut 12 cm from each side.