What you do is tarnish the metals. Go check out some guitar forums for very innovative ways to do this.
Reason for guitar forums, is there's a HUGE business/scam going on with "relic" finishes on guitars....I won't go into the why it's a scam, but you can probably figure out why.
The non blued steel needs to grey, the blued steel needs to brown in certain areas while wearing off in others (and the wear spots should be greyed as well)...if you have cellulose grips, you need to create the conditions to accelerate cellulose rust (cellulose outgassing accelerates corrosion, straight razor collectors often have a box spcifically for cellulose handled razors for this reason, since there's a far higher degree of maintenance needed to prevent corrosion). Brass needs to form a nice patina, but not form green corrosion.
Wood is another matter entirely..I have no idea how to weather wood to make it look old.
There's a lot to consider, but the materials in use are in use in a few other hobbies (knives and guitars) so checking them out will lead to the best advice.