I count 6 general steps in building a frame starting with 1st creating the design. This process includes matching the tubes to your bicycle position and the type of riding you will be doing. A lot of beginners make a full-scale drawing, some use CAD and I use my laser cut and etched fixture that is in reality like a full-scale drawing.
2nd the materials are chosen that maches your design. Some choices make your job easier and depends on how much fixturing you buy or make. For example choosing a lugged bottom bracket shell makes it much easier to get the chainstays the exact length so your rear wheel is centered.
3rd you prep the materials and miter the tubes so they all fit together and match your design. Most beginners miter by hand with files because it is cheaper and faster but I like the absolute precision of a machine miter. You probably could build a frame in the time it takes to set up a Bridgeport to do the job properly.
4th you braze or tig the frame together while during the process you choose a sequence and perform checks that keeps the frame in alignment. Most commonly you do this with the help of a flat surface of some kind. A common way to tack the frame together is to use blocks holding the tubes on your full-scale drawing.
Whether you tig or braze depends on your competency. Tig welding thin steel tubing requires great hand eye coordination. If you aren’t good at tig welding now, I would not use that method building a first frame.
5th do the finish filing to improve its appearance and add bridging and braze-ons. And 6th paint the frame.
Just as fair warning, I’ve been teaching framebuilding classes since 1976 and it is a rare student that doesn’t need a lot of assistance to get things right. It is easy to make big mistakes. Among those students are many that have already taken a class somewhere else or have built frames before on their own. Of course my standard is quality and not just good enough.