I cut my son's & daughter's crickett rifle laminated stock back when they were too small for the, uh, "stock" stock.
I wrapped the general area with painters tape, measured several times and then marked the cut line c a r e f u l l y. I securely clamped the stock to a work table, taking care not to scratch things. I oriented the stock such that i could run along the table (sacrificial) edge with the shiny new fine cut circular saw blade. I set it up with care and made a good cut.
Nowadays I have a table saw and I would use that instead of my circular saw. Using the table saw almost feels like cheating.
Alternately, have a wood shop or gunsmith do it for you. Had an old gunsmith cut down my wife's shotgun butt and mount a recoil pad. Beautiful job, just beautiful. If the stock or rifle is spendy or sentimental, might wat to go this route.