# Be careful regarding any plague of leprosy

"Pay attention if you suffer from leprosy" or "Pay attention if you have leprosy"

# every instruction given to you which the priests, the Levites, teach you

"all of the instructions that I have given you and that the priests, who are Levites, teach you to do"

# teach you ... you will act

Moses here speaks to the Israelites as a group, so these instances of the word "you" are plural. 

# as I commanded them, so you will act

"you must make sure that you do exactly what I have commanded them"

# commanded them

The word "them" refers to the priests, who are Levites.