I successfully made a viable flex pcb on my second attempt.
I started by printing the circuit onto a mailing envelope using my laser printer. Then I taped a piece of toner transfer paper for pcbs shiny side up directly over where the print on the envelope was. This way I could use just a tiny bit of the expensive toner paper and know the printer would hit that exact spot again when I reload the envelope in the same spot.
The print landed right on the toner transfer paper according to plan.
I then sanded with 400 grit sandpaper the Pyralux flat flex PCB copper blank and wiped it off with a alcohol prep pad. These actions clear any oils and oxidation and give more bite for the toner to cling to the board better.
I then taped directly onto this toner transfer paper print the Pyralux flat flex pcb copper blank. No need to even take it off the envelope. Just taped it right over it and fed the whole sandwiched assembly through my laminator a few times envelope and all.
When I peeled back the Pyralux flat flex PCB my laser printer's toner was indeed transferred over to the Pyralux flat flex PCB's copper.
I prepared etchant solution mix of 1 part etchant powder to 4 parts water. I just eyed this roughly and think I did not put in enough echant which causes undercutting of the traces under the toner and slower etching. Lesson learned.
I mixed it in a silicone earplugs container. My aim was a small container to make a smaller batch of the etchant to cut down on etchant used since I'm only doing a very small PCB.
The first board I left etching for a couple hours unattended which was a mistake. It was unusable. A ton of the copper under the toner was missing which is called undercutting. I left it etching for too long which causes this.
The second board came out pretty good. But I used the exhuasted etchant from the first board which was already too diluted and so the results were meh but good enough to use IMO.
Note: the prints going onto the toner transfer paper are not very high quality and sometimes has missing spots so AFTER transferring it to the copper I used a Straedler permanent lumoocolor super fine tipped pen and magnification to carefully color in any missing spots where the laser printer failed to deposit enough toner or the toner failed to transfer perfectly enough. I used stippling method with the pen - just dotting over and over rather than drawing to get max precision for cleanup of the tiny pads and traces on the copper.
Note: I never had to use water to remove the toner paper from the pcb. Just laminating it a few times through my laminator was enough for the transfer to take place and I was able to cleanly peel it away. This meant the toner transfer paper could remain taped to the envelope and be reused indefinitely. I reused it a few times successfully as I dialed in the processes. This is very nice. Saves time for sure.
Note: I did attempt to not sand nor alcohol treat the Pyralux copper pcb blank and toner transfer onto virgin copper blank but it did not adhere well enough so I reverted to the recommended sanding and wiping after all. Was worth a shot to save steps but did not work out.
Note: I used heavy paper setting in photoshop during the print dialogue settings because the normal print settings were kind of messing up for me. I also think this printer is not very well suited for this. My other laser printer has a "best" quality option and did very nice prints but this one is a cheapo I'm using and only has "fast" quality but worked well enough nonetheless for the most part.
Note: I assumed I could use this etchant over and over and over but chatgpt said it gets exhausted and loses efficacy and should only be used once. Some acids people made online you could use over and over but I guess not this type not sure.
Note: the acid etchant I'm using says it only releases oxygen so the fumes I guess are not bad like some other kinds of etchant - correct me if I'm wrong on that though.
Note: I used a q-tip and lacquer thinner to remove the toner after the etching was done.