Quick follow up:
Dead/No pressure: Vacuum, wait (a couple of hours), confirm vacuum, charge on the LOW side with the recommended amount of refrigerant. (A flush is recommended if replacing a component).
Some pressure and unknown leak: Pressurize on the low side. Find a chart for pressure vs/temp and fill to correct pressure based on temp.
Use dye only if you know you have a leak and know what to look for. Don't use sealant or most shops won't touch it. If you did use sealant, don't tell them.