I find I struggle with no effort!
Most will tell you to make line along the tube axis (to align fins against) with a piece of angle, or as Estes says, against a door frame. The Estes jig for this does work, as that's all it really is, a fancy length of angle.
Marking these lines about teh diameter of the tube (so that teh fins are equally spaced about the tube), you either cut out the paper marking guide in the Estes instructions, the Estes tube marking tool, a fin marking guide download from Payloadbay.com , or with 'pencil & paper' geometry..
Lastly, and the hardest is keeping the fins perpendicular to the tube itself (so the fins stick out nicely!) There is the Estes Fin jig (teh grey plastic base one, and teh 'fin marking tool' has a notch at one end to hold fins) or a foamcore / plywood template (as David shows above), or the old Mk II eye ball. I have one better that is easier to make (see below). But all of these work [except the foamcore / ply template] for "typical" fins; fins that don't extend too far beyond teh bottom of the body tube
But all of this comes with practice, and a feel for what works & what just .. doesn't!!
You can also splurge and the the guillotine jig for fin placement & alignment, and there is a really fancy (and $$$) jig out there, which you can dial into 0.5° fine placement!
My fin perpendicularity gauge:
Go to Post #19 (I have a few!)
https://www.rocketryforum.com/threads/madcow-super-dx3-patriot-a-build-thread.140215/
this thread will also help with the conversation
https://www.rocketryforum.com/threads/fin-alignment-techniques-for-big-rockets.167720/#post-2166880
https://www.payloadbay.com/page-Tools.html