I will be happy to help.
I am one of the few ever to have done so.
It is medium-hard, and medium-expensive.
Mine is very-large, very-expensive, by hobby standards.
I am still upgrading mine, 100k€ later, 14 years later, version 5.
Fwiw..
A 25/5 screw is == good for a bridgeport sized area.
30 mm linear guides = good for a mill in this size.
Use heavy preload blocks.
30-35" in x or == 800 mm x, 300 mm y, if using a C frame.
A double column or bridge mill is 8x more rigid per volume.
Heavier is better.
10x heavier than planned, usually, == right. Yes.
Modern machines are all stressed-skin lightly loaded structures of == 15-10 mm cast iron skin thickness on castings.
Make every part "bigger" if you can.
This is called section size.
Example.
Original industrial Bridgeport mill had about 200 mm D of round casting for the mill head ram, in a C frame.
Late, excellent, german deckel FP4(3) mills had about 250-350 mm, 250x350 mm.
The deckels were ==10 more rigid == 10 better.
Now, I am using about 2-3 cm thick tool steel for everything.
4-5 cm for head/vibrating parts.
My "ram" is 240x200x20 mm box, 35 mm linear guides (hiwin). 20 mm thick.
40 mm thick plate for spindle mount, 200x200x40 mm.
Fwiw..