Hi kaian: you can do this in a one-step (rustproof only), or two-step (neutralize and rustproof) process. You will need to get:
-duct tape
-rust neutralizer liquid (for the two-step process) Any automotive store should carry it. Look for names like Rust-Chek or Rust-Stop, etc. Get a can of liquid; spray cans won't really work here.
-a litre (or pint ) of boiled linseed oil. Any hardware store should have it. Alternately, you can buy any of several bike-specific frame treatment products like Frame-Savr etc. Linseed oil works as well and is usually much cheaper.

-When the front fork and BB, are removed, tape over all those openings completely with duct tape. Also tape over the hole in the chainstay bridge (the small tube connecting the chainstays right behind the BB)
-poke a few small holes in the duct tape at all those locations with a safety pin or something similar
-locate the tiny frame breather holes (usually on the inner side of the chain and seat stays near the rear dropouts, and at the top of the seat stays where they are brazed to the frame up near the seatpost). Carefully clean them out using the tip of a safety pin on thin piece of wire. Don't tape over them.
Two-step process (go to step 2 if you are only rustproofing, and not neutralizing as well):
1)- pour a litre (or pint) of rust neutralizer liquid into the seatpost opening and insert the seatpost back in and tighten the bolt. Then simply rotate the frame slowly by hand, end-over-end and side-over-side several times (slowly) to allow the neutralizer to reach and treat all the internal tubes and joints. Watch for the liquid seeping out the holes you poked in the duct tape, and the frame breather holes. This will help tell you when the liquid is getting to the right places.
-remove the seatpost, and allow all excess liquid to drain out. This may take a while. Rotate the frame occasionally.
-let the frame dry overnight in a warm location.

2)-remove the seatpost and pour a litre or pint of boiled linseed oil into the frame, insert the seatpost again, and use the same slow end-over-end and side-over-side procedure several times to coat all the internal surfaces and joints. This works best in a warm location. When you're sure you have coated all the internal surfaces, remove all the duct tape and drain the excess oil from the frame. Rotate end-over-end etc often until ALL excess oil has drained. Be patient. It may take several hours or even overnight for all the oil to drain out. Allow the frame to dry well. This can take a few days depending on the temperature. Rotate the frame occasionally.
-clean out those tiny seat and chainstay holes with a safety pin so the frame will breathe properly.
-wash all excess oil off the frame, and off the BB threads only, and you're ready to rebuild.
Good luck!

~Sherry.