A flat surface is the best and most accurate tool to build a frame. Like Steve said, since you might not build another frame it makes the most sense to use that as the base for holding your frame. The simplest way for most builders without a mill and/or lathe is to buy aluminum tubing blocks from Paragon Machine Works for $15/20 each and lay them on top of your drawing on your flat table. In some sequence you spot braze the tubes together and check that the frame is still in alignment (and adjust as necessary) and finish brazing free in your bench vise or Park stand. For the greatest accuracy that is a back and forth process.

Of course as an alternative you can attach tube holding blocks to some kind of 80/20 fixture. The problem is that 80/20 is not as accurate as your flat table so what’s the point? Often hobbyists don’t have access to a flat table so the fixture becomes the alignment reference. The purpose of a fixture is to increase accuracy and setup time and look cool to admiring visitors. It is not something to do a full braze in because it restricts moving the frame to the most advantageous position while brazing.

One of framebuilders most basic but necessary tools are wood blocks to hold the frame in your bench vise. Wood blocks have the advantage of having the right amount of holding vs movement ability. The Bicycle Academy in the UK sells them. I have a set for sale too. Sometimes an amateur will use the aluminum Paragon blocks instead. Those Paragon blocks are also useful when either machine or hand mitering. You will have to decide if it is better to spend your time making blocks or your money buying them.

You will get a variety of suggestions on methods to make your frame. For example some builders like to pin a frame before brazing to help hold it together and keep it in alignment when brazing. It is a successful method and the way I learned but isn’t how I do it now. You’ll have to figure out what might work best for you.