When I was young like around 7/8 years old, my dad used to create R4, and today i'm just wondering how the fuck he did that, if anyone know maybe you can tell me ? thanks !
Setting up an R4 flashcart or any flashcart for that matter isn't complicated, just follow the instructions. Now if you meant create from scratch then the process isn't easy. First off you would need the technology to make this stuff work, so the chips, PCB, etc. next would be the software, most of that is done with libnds and devkitpro. Lastly would be the shell with sticker. Here is the thing though, it really wouldn't be a good idea to make an R4 flashcart as we already have a couple that work well already, you would simply be wasting your own time making one. Personally I would just make the software for existing flashcarts, there is a lot of potential with these devices that people today aren't really taking advantage of.When I was young like around 7/8 years old, my dad used to create R4, and today i'm just wondering how the fuck he did that, if anyone know maybe you can tell me ? thanks !

I don't want to make one, just know how to make one ^^Well, he probably did it in a factory, because doing those homemade are pretty crappy, and you would need to have custom PCB, chips and even custom shells to make them in good quality… so maybe he just assembled those.

Do you mean this:I don't want to make one, just know how to make one ^^
I remember saw a tuto online but can't find it
thank youI have never seen a homebrew DS flash cart.
The old passmes had PCB schematics released ( http://dspassme.jzdocs.com/ ), as did things like DSserial ( https://web.archive.org/web/20051210071447/http://natrium42.com:80/projects/serial.html ), https://web.archive.org/web/20160910093752/http://www.reinerziegler.de/ has a bunch for the GB/GBC and GBA but I never saw anything for the GBA/DS.
There was a R4 clone kit sold to various Chinese electronics concerns so people could make them (there is a reason there are a thousand R4 clones out there) but I never saw it released to the public (the going rate was a few thousand US Dollars, about normal for such a kit).
Beyond that I have nothing, especially not in the timeframe you speak of. It would have been technically possible I guess if he was a decent electrical engineering inclined type but certainly not a project to follow along to. Such a thing was desired by a few in the community as well, including those playing with wood RPG/AKAIO and other such projects, so yeah.
If you did really want to make your own then you probably want to spend some time reading the passme stuff, http://problemkaputt.de/gbatek.htm (he was the guy that first figured out a lot of the DS slot stuff and kicked off that whole thing), https://hackmii.com/2010/02/lawsuit-coming-in-3-2-1/ and you might be able to find the akrpg/woodrpg source around somewhere (depending upon how you make the flash cart you might also have anti piracy to deal with if you want games https://gbatemp.net/threads/open-ds...and-hardware-removal-database-project.355223/ ).
