Very simple one that Iím sure a lot of you already useÖ

Many moons ago when I did basic workshop practice, we were taught to use a square when hand tapping holes - to ensure the tap stays squar to the workpiece. Good in theory but in practice it sucks.
These days when I have to hand tap I use a guide to get the tap started square.

Basically drill a hole of the same size as the thread - ie for M5 use a 5mm hole - in a piece of scrap stock at least 3-4 times as thick as the hole diameter.
Sit it flat on the work over the hole to be tapped, poke your tap through and as long as the hole in your block is square the tap will stay square.

Hereís one I knocked up today for some M5 holes.
Small round offcut, faced and recessed the center a bit (stops the guide sitting up on any burr around the hole, then drilled 5mm through.
Hit it with a number punch so I know what size it is for next time.