The proper cup and crown race tools are worth having; using the hammer and screwdriver bodges are wrong and can damage parts, but IMO the cutting guide is not. If you're cutting it at home, you can afford to take your time and do it properly.
You can either cut then tidy up with a file or put an old headset on the steerer as a cutting guide, which will also clean up the threads when you unscrew it.
+1 for this. Great idea to use an old headset locknut (ideally steel) as a cutting guide / thread chaser.
I've recently invested £24 in a crown race fitting tool as I once broke an alloy Campagnolo Record one
and spend a long time (and about £24!) finding replacement and getting it fitted at a LBS. I have used an old stem to bash crown races into place but it took forever!
As for the headset cup fitting tool, I made one for a fiver which has never failed me (the cost gave me enough for two so I gave one away to a helpful friend) and the cup removal tool is also easy and cheap to make out of a soft pipe with a cross cut into it.
This is just my opinion of course, and I honed my skills on dozens of cheapo bikes before attempting anything on a nice machine with delicate parts.