The next step, honestly, is to choose a plan and purchase it. They explain everything that needs to be done with what you source. There are also tutorials on the website on how to break down a bicycle, and those deserve a read too.
Unfortunately there's no free ride here (no pun intended). To do this, you're going to need some basic tools that can't be avoided: A welder, a grinder, basic hand tools, and some files. I got my welder for on offerup $15 and fixed it up for another few dollars. $50 at Harbor Freight will get you grinding and cutting disks, a grinder, a file set, some welding rods. Another $25 at a welding shop will get you a helmet. That's under $100 for all needed tools if you shop right. You need a place to work and somebody to help you if you're at risk of having a seizure while welding and grinding etc. That's dangerous.
Then there's $20 for two plans or $36 for six. Still possible to be under $100 at this point. Steel will cost you around $50-75 depending on what's available to you, give or take. That and a few free or almost-free bikes will set you up for success. You can do the whole thing for under $300 for sure, and possibly under $200. But it will cost that, unavoidably.
Another avenue you could explore is that of borrowing tools. Countless people have welders buried under junk in their garage, or have a workshop that hasn't been put to use in a long time. Ask around and see. But no matter what, you're going to have to bite the bullet in some regards and start learning about the practicalities of making this project happen on your own. It's a non-trivial thing to build a bike from scratch, but it's rewarding- and fun!