Further to the link above, I will summarize for you...

- Install the Piston Seal.

- Install the boot on the caliper side first.

- Now slide the piston in slowly, making sure it sits squarely on the seal (i.e., don't fold the seal over!).

- Once the piston is pushed all the way in, you will hear a faint "pop", that is when the boot's inner lip slides onto the piston's groove.

__________________
1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT