Place a good amount of cutting fluid on the reamer.
Go very slowly (clockwise) you can always cut more metal off, but if you go too far you can not put any back. Just go
slow and take your time. Measure with the GO gauge frequently and do not forget to clean the chamber and reamer of shavings
when testing headspace.
After you cut and are ready to test clean the chamber and place the GO gauge in the chamber and the bolt on the rails.
If the bolt does not fall past the bolt lug and rest on the reciever, you need to repeat the cutting steps untill it does.
Finish reaming the barrel can take sometime, but as long as you take your time the process is idiot proof.
When the bolt falls completetly off the bolt lug and rests on the reciever (see photo below) you are done.
Gratz you just finished reamed your M1