Step 8: compress piston
Since your new pads are obviously much thicker than your worn-out pads, there's not enough space to put your new pads in place.
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.