What components do you have available?
Figure out a powder and a bullet you want to start with. Then start on a ladder test to see which shoots the best.
A ladder test is where you start with a minimum loading and go up from 0.2-0.5gr every 5 rounds. Not exceeding max of course. Some guns can show high pressure signs before a book max load is reached. Shoot for groups and see which powder charge gives you the best results. Then fine tune from there.
Also, when seating for the first time, use your chamber as a guide and seat the bullets just off the lands. My 243 has a long chamber and requires me to seat my bullets further than the books suggest.
I don't shoot a 204, but the process is the same for any rifle. Each rifle is different.