Step 8: Compress piston
So, you need to compress the piston back into the caliper.
There are many ways to do this; probably the safest is to use a C-clamp around the piston and the back of the caliper.
I couldn't do this, so I used a GENTLE prying technique between the caliper and the piston.
Be careful not to scratch, mar, or bend your brake disc. And be careful to exert pressure *straight* on the piston, as much as possible.