What he said !
Start with the barrels and match them to the gaskets as per the posts on the subject . Coz I have recently used the photos off this site to try exactly what you are doing , I can understand the fear of stuffing up , the geezuz I am holding this revved up tool with a cutter in it , am I going to make a complete F%$K up of it .
Start on a barrel and go quietly and hold you tool firmly . Buzz into it gently to the marks on the barrels for a couple of minutes , then go back onto the computer and look things up again .
Really becomes quite addictive
Good Luck and stay confident . Cases after barrels , MJs photos there are gold .