Before opening the guard make sure the machine is isolated from electricity (so it doesn't accidentally turn on)
Open the guard and check that there is nothing already in the chuck.
Collect your chuck key and drill bit.
Using your hand, open the chuck (by twisting) until you can place the drill bit between the 3 jaws. Now tighten the jaws around the drill bit.
Use the chuck key to tighten the chuck even more. Make sure you remove the chuck key and close the guard before turning on the machine.
Now you are ready to check if the drill bit is straight.
Check the drill bit is straight by turning on the machine. If it looks like its wobbling then start this guide again, you need to make sure all 3 jaws are holding the drill bit.