if you can, pick up some range brass in the same calibre that you want to reload and practice on that first. Clean it and experiment with your full length resizing die so you get to understand how to measure and bump back the shoulder, this might take some time before you get it right. Goes without saying - do this without inserting live primers, leave that for later when you can resize your brass properly. You will need a caliper (I suggest digital) and comparators that screw on to the caliper blade eg Hornady, Sinclair with inserts to measure shoulder bump and later, bullet seating depth. Search this site using key words, you will find all the info here eg "seating depth"