i have scars that are still here over a year later, big huge ones that are a centimeter thick, but also supper supper skinny/tiny ones. razor blade ones, especially from good razors i've found are reallyyyy good. they've caused my biggest cuts, they go through skin like paper. razor blade ones only stay if you go really deep and quick (when i say like paper though i mean it, go light at first and then you can test out different strengths). for tinier cuts staying, i recommend something more dull and going slow actually. slowly dragging something still somewhat sharp but also somewhat dull can cause scars to stay.
final thing this is horrible horrible don't try but if you do be extremally careful. so usually when i would cut, i would disinfect it, clean it put bandages, etc etc. i've found though, when i don't take care of cuts as much, keep picking at the scab, etc, those are usually the scars that last longer. my worst ones have even gotten infected and i think those will probably stay the rest of my life (even if i didn't ctb lol). its really bad to let a cut get infected, know the signs and make sure you can take care of it when that does happen.