I once installed headset cups with a heavy coffee mug and a towel to protect the cup. Bam, bam, bam... it worked!

That said, I was a LBS mechanic at the time and the headset press was 10km away at work... I was just too lazy to go there and do it.

Let a competent LBS do the job for you. I'd err on the side of caution and have the fork cut a little long (i.e., use and/or buy 5mm more spacers than you think you need) then you can move some spacers above the stem until you're happy with the stem height. At that point, return the the LBS to have the extra steerer tube trimmed off.