This is a given and nobody should have to tell you to do so. Working under an unsupported vehicle is for morons!
Now, before you start pulling off wheels, I recommend you situate the vehicle so there is at least four feet of open space around every location you’ll be working. I installed this lift on an extended cab and in order to comfortably do that I had to keep the bed outside of the garage while I worked on the front. If you have a large garage then you’re luckier than me. I don’t have a large garage so this was my only option. Unfortunately it meant the back of the truck would “hang outside” for a few days. Make sure your neighbors won’t get upset if you do this especially if they live next door and their house is on the market! (oops)
Also, it is not necessary to get the entire vehicle on jackstands at this point. Just get the front lifted up. The rear tires will function as adequate make-shift jackstands for the time being.