Blind rivets -- commonly called pop rivets because of the company that popularized the style -- are used to secure to pieces of material together. You push the rivet through holes in the pieces and squeeze its mandrel, a long needle-like piece, with a rivet gun. Sometimes you need a washer on the back side of the rivet if the pieces being joined are thin and could potentially tear when the rivet is squeezed.
Align the holes in the pieces of material to be riveted.
Insert a rivet of a suitable diameter and length into the hole. The rivet must fit tightly in the holes and be long enough to account for the thickness of the materials being riveted and the thickness of the washer. The rivet kit will have a table showing which rivet to use according to hole diameter and material thickness.
Place the rivet washer on the rivet head protruding from the pieces being riveted. This way, the two pieces being riveted are sandwiched by the rivet flange on one side and the backup washer on the other side.
Open the rivet tool's handles all the way. Insert the rivet's mandrel into the nosepiece of the rivet tool. Squeeze the handles until the mandrel breaks off the rivet head. You may need to hold the washer in place by hand until the rivet starts to mushroom -- deform -- during the initial squeeze.