Chad, your question/quest is pretty broad without some kind of guidelines that can help us reach your goal. What is your budget and how much prior experience do you have might be starter questions. It won't be very long until I've been making frames for 50 years and I can tell you there are many approaches to how to make them. While many can work fine, not all of them work well and our personalities draw us toward different approaches. How you start out can shape how you do things in the future.
The cheapest and easiest fixture is to lay out Paragon Machine Works aluminum tube blocks on a full scale drawing on some kind of flat surface. The frame is not brazed in the fixture but rather spotted together and brazed free.
I'm an American that learned in the UK (because that is where the knowledge was) and many of those builders at that time used simple methods. Ever since I got back I've refined a method that was popular over there. It is kind of like the fixture from Temecula in which an outer "picture frame" supports the pieces that represent each tube. I've refined that concept so almost everything can be set by marking on the fixture so I don't have to use protractors and rulers to set angles and lengths.
The approach I use to design a frame is to start with a person's bicycle position (their seat/handlebar/pedal relationship) and orient the tubes around that found position. It is a bit different than the racing approach where the rider is expected to fit the design. My fixture allows me to fit the frame to the rider. This can be especially useful as a person ages and his riding style changes.
These are laser cut and etched out of stainless in Ukraine.