How to set the origin and remember that point

Go to menu "Origin setting" and enable the desired origin (e.g., Origin 1)
Press <Shift> + arrow keys to move the laser head to the target position
Press <Origin> to record the position, then press Enter to save
Select which origin to use at startup under "Start origin"

On my machine, I first use the arrow keys to move the laser head to the desired position. Once it’s in place, I turn off the X, Y, and laser controls using a separate switch. With the switch off, I use the arrow keys again to set the current position as “0” and “0” — the carriage doesn’t actually move during this step. After that, I turn the switch back on, and the carriage stays exactly where it is.

I’ve also noticed one important detail: after powering on the machine, I have to wait for the Ruida controller to finish its startup process before turning on the X/Y and laser switch. If I turn it on too early, the carriage will automatically move to the front-left corner instead of staying in place.