Modding XGM is much like modding vanilla RTW or any other mod with a few exceptions resulting from the way in which the -mod switch works.
1. Your game folder is probably something like C:\Program Files\Activision\Rome - Total War\
2. Modified files should go into the xgm\data folder not in the original data folder.
3. You do not need to unpack the .pak files to change UI cards. Just put new UI cards in xgm/data/UI.
4. The engine will usually check the mod folder to see if a file is located there, but it will not do this for .CAS files (models), or .SPR files (sprites). You need to specify the location of these files in the relevant text files (for units the file is descr_model_battle.txt).
Preference settings for XGM are stored in xgm\preferences\preferences.txt. There are several settings that you might want to change manually to suit your own tastes. You can use notepad to make the changes.
1. Fatigue is turned off by default. If you want it back on change the fatigue line to: FATIGUE:TRUE
2. Green arrow markers are switched off by default. You can switch them on with: DISABLE_ARROW_MARKERS:FALSE
3. I usually switch off unit banners: SHOW_BANNERS:FALSE
Most of the other possible changes are self-explanatory.
Spartan Royal Guard
Quite a few peole have asked how to reduce the recruitment time for the Spartan Royal Guard. The short answer is you shouldn't do it. Historically there were only 300 men in the Royal Guard, and if you make it easy to recruit an uber-unit like this you will make the game too easy. Wait until you have an army barracks and recruit Spartan Perioici instead. :)
OK, so if you really must change the recruiting time, you need to edit export_descr_units.txt. Find the first entry for spartan hoplites (not the one with xgm on the end), and change the first number on the cost line. The second number is the price. Here is the entry with the recruiting time in red:
type greek hoplite spartan dictionary greek_hoplite_spartan ; Spartan Hoplites/Royal Guard category infantry class heavy voice_type Medium_1 soldier greek_spartan_hoplite, 50, 0, 1.5, 0.3 mount_effect horse +4, camel +4 attributes sea_faring, hide_forest, can_sap, very_hardy formation 1, 1, 2, 2, 5, square, shield_wall stat_health 3, 0 stat_pri 12, 6, no, 0, 0, melee, blade, piercing, spear, 25 ,1.0 ; Hoplite stat_pri_attr no stat_sec 0, 0, no, 0, 0, no, no, no, none, 25 ,1.0 ; no weapon stat_sec_attr no stat_pri_armour 9, 8, 5, metal stat_sec_armour 0, 0, flesh stat_heat 2 stat_ground -1, 0, -2, 0 stat_mental 30, disciplined, highly_trained stat_charge_dist 30 ; Foot stat_fire_delay 0 stat_food 60, 300 stat_cost 30, 8000, 520, 90, 130, 2400 ownership greek_cities, romans_brutii
If you do change the recruiting time it might be a good idea to edit export_descr_buildings.txt as well and move the recruiting line to the Royal Barracks (don't forget to delete it from the government buildings).
Battlemap Movement Rates
Battlefield movement rates are controlled by xgm\Data\descr_battle_map_movement_modifiers.txt . You can change the movement rates back to the vanilla speeds just by deleting this file from the xgm folder, or you can modify the rates to suit your own tastes by changing the numbers for each terrain type.
Ranged Accuracy Settings
Units with ranged weapons all have reduced accuracy. To return them to the original settings open [Rome]\xgm\data\descr_projectile_new.txt and remove the 'accuracy_vs_units' lines from the appropriate projectile entries. For increased or decreased accuracy higher numbers are less accurate, lower numbers are more accurate, with 0 being the original default value.