When you've completed all these steps to replace your SSD, don't despair if the MacBook Air shows a flashing folder with a question mark when you first power up the MacBook Air.
- Power off the machine, then keep the option key (= Alt key) pressed down, power on the machine again, and keep the option key pressed down until a prompt appears.
- If you've set a firmware password, then type it in at the prompt
- You should now be prompted for a hard drive to boot from. Select "EFI Boot"
- The MacBook Air should now boot to a window showing "OS X Utilities"
- Click on the at the top left, then select "Startup Disk..."
- Select your SDD/Hard drive, and restart.
The 8mm and 2.5mm are the length of the screws. One pentalobe P5 screwdriver suffices for all the screws (P5 is implicitly the size of the pentalobe screw heads).