Choose length of the cable. Size should be on the inside of the tongue of your boots. For the H2 BOA systems with a hole in the dial knob's "O" you need a small allen key to remove the knob and be able to replace the cable. For M3 BOA systems you need the torque version (grey). If you don't have this tool, you need to buy this too (available in this section)
Search art code 19918 BOA Mounting tool in this section.
Advice: make pictures of your demount with your mobile for later reference. Some BOA sytems are more complicated then others. Search youTube for instruction movies.