First I should probably link the following as it is better than both those links, on the first link it looks like the guide maker did not create a cart using the guide either.
http://reinerziegler.de/GBA/gba.htm
That link looks like it covers the HCl using toner transfer method. Not my favourite method of making a homebrew PCB (a milling bit and a CNC would be my first choice but those are not terribly cheap) if I am going with chemicals but it works if you do it right. This is not the place to start a Ferric Chloride vs Hydrochloric discussion though.
Saving. No that homebrew cart does not save or have the function for it. There is an as yet untested build with save support on the link above as well.
You can flash things in many ways but in this case if you do not want to do any extra programming then it will want a GBA or GBA SP and a so called multiboot cable. You run this on your PC and connect it to the GBA, it then boots like it thinks it is connected to another GBA with a link cable (or a GBA-GC linkup I guess) and with that you can send the relevant flash commands.
If electronics interests you and you want to have a project to keep your interest then yeah try this. If you want a useful flash cart then this is not the way, at least not without modifying it and that will take at least some electronics knowledge -- though you can probably find something you can wire up directly to power and pins without any intermediate logic/voltage handling/crazy fine tuning on passives. If you are thinking this might be a cheaper way to a flash cart it is not -- if you already have the gear or need it for something else then OK but buying it for just this is not a cheap method.
