have the par files in the same folder as your .rar files and open up any of the par(or .par2) files. it should automatically go through all of your .rars and analyze them. once it finishes analyzing the rars it will look for other par files in the set and it'll tell you if it has enough to fix it. after that(assuming it has enough blocks) click the repair button and it'll repair your rars. the process is automated so you do almost nothing. it should be noted that analyzing and repairing rars is a fairly intensive process so if you have a slower computer prepare to be patient.