I've just replaced a lower cup on a bike. A wonderful bastardization of a 50mm headtube and fitted a Hope type stepdown 1.5" to 1 1/8", which is 49mm. Bought 1/2mm alloy sheet to cut a strip to shim it with.
I didnt use a press and only have the threaded rod and several big washers, and tbh it was more a pain int he bum trying to align it, so i tapped it in using a hammer and a bit of ply
My technique, if you can call it that, is to sit with the headtube in my lap. place the cup in place and give it a bit of a dunt to at least get it started.
This starts it but it will be misaligned, so you take note of the highest bit, and tap that down, and just work your way around, one side to other till its about home and straightish, then with the ply as a softener, give it some good hard whacks
By sitting it on my lap its easier to hold it in place with one hand while holding the softener block roughly in place and swing the hammer at the same time. Trying to balance everything while its up in a stand theres more chance it will bounce on the first hit, drop to the floor and roll under the fridge.
