Well, the easiest way is to true the action on a good lathe. But, if you don't have $5000+ to drop on a piece of machinery, or don't have the training to use it, you're still not out of luck.
There's systems out there that allow "blueprinting" a Remington 700 action without a lathe. They are made by Brownell's, Manson Precision, and PTG.
I've not tried the PTG tools myself, but they look to be of good quality and a great design..
The cheapest route to go is the Brownell's tools. This is a large, 1.5" round cutter that accepts a pilot that's .700" in diameter. The pilot centers itself in the boltway/lug faces and the cutter "squares up" or re-cuts the receiver ring (front ring of the action that is against the recoil lug). The problem here is that more often than not, the front rings on even brand new Remington actions are larger than .700", allowing this pilot to be able to wobble a bit. If you can have a local machine shop make you a pilot that runs the full length of the action, this would probably work very well. Even without, you can "tighten" up by using a 1/4"-28 set screw in the front action screw hole to keep upwards pressure on the pilot, keeping it snug against the upper lug.
Next you have the Brownell's bolt lapping kit. I use this kit myself, and it's a dandy. It screws into the front of the action, where the barrel would go, and is spring loaded. Then, when you insert your bolt (minus firing pin assembly and ejector), it is pressing rearwards on the bolt face equally - so by putting a dab of 480 - 800 grit silicon carbide lapping compound on the rear of the bolt lugs, then lifting the dropping the bolt handle a couple hundred times, the lug faces get "matched" to the lug seats.
The Manson kit takes all this a step further. This kit uses tapered bushings to combat the differences in action boltways, and the tools center in these bushings. First, you have a tap, which re-cuts and squares the action threads (both a standard size kit, and also a .010" oversize kit that comes with a reamer). The next tool actually re-cuts the lug faces to be square and straight with the action ways. This kit runs around $500, and doesn't come with the receiver ring cutter, which, purchased separately, needs to use the pilots to center itself as well while it trues the receiver ring (face).
Another Manson tool is the bolt face cutter. This tool also threads into the receiver threads, and centers a facing cutter to by turned via drill, press, or milling machine, and re-cuts the bolt face perfectly flat and square (most factory actions are conical rather than flat).
The PTG kit, I've never used - but it is very similar to the Manson kit except for it comes with several sizes of bushings rather than tapered ones - so you can fit the EXACT size bushing to your action - a feature I really like.
The bolt facing tool looks to work just like the Manson tool, and is a might cheaper as well!
Good luck, and happy shooting!
Also, forgot to add: If you want to install a new barrel, you will need a lathe to use a barrel blank... because it will need to be threaded for the action, and chambered... But again, if you don't have a lathe, buy a "short chambered" and pre-threaded barrel - Shilen makes outstanding ones! These comes pre-threaded for the 700 action, and the chamber is cut, but cut short, so when you install it on the action, you won't be able to chamber a go gauge, much less a real round. Here you use a finishing reamer in the proper caliber, and carefully cut the last couple thousands of the chamber until the bolt just closes on a GO gauge. You do this after the barrel is installed on the action, and use an extension T handle to hold your reamer. Even smiths' with a lathe use this method - I do.
