Applying Brake Cleaner is easier with a dismounted wheel. You can remove the brake pads for a more thorough cleaning, but you don't have to. You spray Brake Cleaner directly onto both sides of the brake disc. Let it soak in for a few seconds, then rub the disc with a clean, dry cloth.
Then also spray Brake Cleaner directly onto the brake pads. Let it soak in for a few seconds and repeat this. Then wipe the brake pads with a clean and dry cloth.
A piston that does not return properly can also be the cause of a squeaky disc. In this case, you can also use Brake Cleaner:
Disassemble the wheel and the brake pads. Put a plastic block in the brake calliper so the piston does not come out too far when you pull on the brake. Remove the plastic block and spray Brake Cleaner on the pistons. Then wipe off the piston by sliding a dry cloth through the caliper. Spray a little Bike7 Shine & Protect on the pistons and rub again with a cloth. Push the pistons back using the plastic block, fit the brake pads and wheel again and re-centre the brake pads.