As Semimmortal mentioned essentially what you're doing is using either AltanaViewer or Model Viewer to bring up the specific .dat you want to change and then by using the given location and name of the .dat, swapping the old one out, using the name of the one you swapped out in place of your mod (renaming fancymoddedshoes.DAT to 10.DAT as an example) so that the game thinks it's loading it's normal set of .DATs while loading in all the modified ones you'd like to use. It's just replacing and renaming files.
The important thing to remember is that every race and every sex in-game has separate sets of .dat files with the exception of taru due to being giant heads atop barrel bodies with arms and legs. :3 What this means is if you use one of the Viewers and you're looking to switch out elf male PLD AF Body and you're looking at an Elf Female wearing it, you're going to switch the wrong .dat. This also applies to weaponskills; if you're watching a Galka try to throw his short *** leg up for a Raging Fists WS the .dat that is displayed is only for a Galka's Raging Fists and no other.
Here's an example using Altana:
Wow that's a huge example. lol The highlighted area is what's going to tell you what .dat you're looking at based on the very last thing you changed. In my case the last thing I changed was the body to Jujitsu Gi, so it's reflecting that the Jujitsu Gi .dat file is located in ROM/37 and named 119.DAT of my FFXI folder.
Just ask if there are any questions or confusions!