The easiest way is to use a touch plate like this
You can also home your machine then move to a known location like the front left corner of your material and then place your bit on the tip of the material on this corner
You can then zero out all your axis and as long as you don’t turn off your machine you can always go to 0,0,0 and set your new bit at that point. Remember if you change but sizes the bit zero will move by 1/2 that bit size in all directions.
The best advice I can give you is by the zeroing block. It will help remove a ton of frustration