Well, the basic process of setting it up for use is the same for all platforms, the only difference being the way inputs work in the settings menu (stuff like button swaps(A/B vs Cross/Circle vs Enter/Escape (although this can still be used using the OSK (except for the PSP build, which doesn't have a touch screen nor a mouse to use it) or physical PC-compatible keyboard))).
Although usually the XT doesn't need much more than the ROMs (and required floppy/CD-ROM images to actually boot it). Although MIDI emulation on non-WIndows platforms requires a soundfont (sf2) as well.
The remainder after placing the ROMs in their correct locations is the setting up of the emulator itself (which can be done entirely from the settings menu when opening it during startup of the emulator).
It already mentions all basic steps required to get the emulated machine booting properly. Is there something missing?
I've added some extra information here and there on the machine specific parts and an explanation of some required i430fx/i440fx settings to set inside the BIOS menus when pressing delete during the POST. It's the same with the real machines, but that's got nothing to do with UniPCemu itself (the exact same is true with other emulators like qemu etc.).
@Ophelia_Later@selever so when you apply a theme, it copies the files to home menu data, they stay there forever until you change to another theme, the same thing also applies with theme shuffling afaik so you can have 10 themes stored in the home menu data at one time, but once you manually change theme, it's gone