Next you should use the left joystick to center the screen both vertically and horizontally, and in this work you must of course use the DOWN button to check current results on the physical vmode.
The intention is that this tweaked mode should fill out your screen height pretty close to what the NTSC screen does in NTSC mode, and with 4:3 proportions set the TV should show almost exactly the same picture as for real NTSC mode, except that the physical mode is 480p, so there will be no interlace flicker.
In practice though, You may get some black borders above and below the effective screen, partly because TV sets vary in this, and partly because the game sometimes affects some GS register in ways we can't compensate for.
As for the tweaking joysticks, sometimes trying to adjust one axis alone will also affect the other, so you need to correct that then. And there is a calibration trick you can do to ensure that the sticks work as well as they can do. Before you choose the base resolution to tweak from, rotate each of the sticks in two full circles at full tilt. This will cause the stick sensors to be calibrated, making it easier to achieve good control. Only when this is done should you choose your base resolution (pressing 'Square' once for 480p) and start tweaking it for real as described above.
This method works fairly well for FFX, but there are some other games that it will not work for, in most cases because those games require an interlaced video mode.
When you are happy with the effects of a tweaked vmode you can save it to the user-mode array, by using R2+LEFT or R2+RIGHT to choose the array index between 0 and 15, and then press R2+UP to store the current video mode in that array slot. And to ensure that this array is available in future you can save it (and all other settings) to the MC by pressing R2+L2.
In future sessions you can reload vmodes from the user-mode array by again setting the proper array index as above, and then using the command R2+DOWN to load the stored vmode to the current preset values, and then you can activate that vmode by pressing just DOWN as usual.
Slot 0 in the user-mode array has special importance. If you store any vmode in that slot, then the GSM program will automatically use that vmode on entry into the program, without you having to give any commands.
Best regards: dlanor