You will need RPFM + a basic knowledge in db table.

For reference, please also grab this mod: Warband Upgrade Extended. This mod provides enlarged UI panel + 20 race-unique UI categories + 2 empty UI tabs to work with.

For compatible unit examples, you may check out Lily's Bretonnia Overhaul and its unit pack.

These example mods are more explanatory than this guide :-)
If you encounter any issue or question, then continue reading.
If you don't find answer below, throw me a question in comment section.

The simplest way is to add to existing upgrade groups. In this case, there are two tables involved:
table 1: unit_to_unit_group_junctions_tables (put your unit into a unit group)
table 2: unit_upgrade_groups_table (name the unit group)


Example 1:

To add units to existing chaos groups, all you need is table 1:
1. find a suitable group from the CA db in table 1;
2. create a new db under table 1 folder, and add in the unit name | unit group.

Say you made another exalted daemon, called my_cute_daemon. The proper group from table 1 would be wh3_main_chs_daemonette_exalted.

So, create a new db under table 1, name it uniquely (e.g., cute_table), and add the following row to the new db:
my_cute_daemon | wh3_main_chs_daemonette_exalted

Done! Now the upgrade will appear in game.



Example 2:

To add units in non-chaos race, locate the race db under table 1 in Warband Extended Mod, find a proper unit group, and add your unit id | group in a unique table 1 db.

I will use what I did for Lily's Bretonnia Overhaul as example: it has a new longbow variant called zayli_royal_longbow.

Locate table 1 folder in the extended mod, and open TLA_upgrade_brt db. Find the proper group you would like to add to. I used the yeomen archer group there, called "TLA_brt_elite_bow".

Next, in your unit mod, create a new db under table 1 folder with unique name (say Zayli_brt). Then add the following row:
zayli_royal_longbow | TLA_brt_elite_bow

Then, go to table 2 folder, and create a db of same name (Zayli_brt). Then add the unit group
TLA_brt_elite_bow
into the entry. (This step is needed so your mod do not throw error without the Warband Extension mod.)

Done! Your unit mod will work by itself; and with the upgrade mod subscribed, there will be an upgrade path to your unit.

You can find more examples in Lily's main Bretonnia mod and the unit submod.

Now, if you would like to create a new group and / or new upgrade path (rather than existing ones in vanilla or my mod),

you will also need
table 3: unit_upgrade_group_ui_infos_tables (specify the location and UI tab category of each unit)
table 4: unit_upgrade_group_ui_links_tables (specify the arrow link between two unit groups).
table 5: unit_upgrade_to_unit_groups_tables (specify the base and target group for upgrade).

You will need to create new group via table 2, associate your units to the group in table 1.
Then, add your group ui info in table 3, create new upgrade link in table 5, and specify the link directions in table 4.

If you are doing unit variant swapping, then table 4 is not needed.

If you are looking into this, I believe you are an db expert & can figure out the remaining on your own by looking at my mod & how it works in game --- or ask me here in this guide if you have any trouble. Again, my main mod and Lily's units submod provides perfect examples on how to add units.

In the table 3 (the ui_infos_tables), there is a category for each entry.

Those categories are specified in
table 6: unit_upgrade_group_ui_categories_tables.
In CA's database, it only has 5 categories for each chaos god.

If you specify a new category in Table 6, it will not work --- because those categories are also linked with the UI tabs in warband_upgrades.twui.xml.

There are three ways:
1. Use the 5 chaos god categories (undivided, khorne, etc.)
2. Use a new category from table 6 of my warband extended mod.
3. Create your own twui file.


Method 1:
Easy to work with, compatible with my mod, but not necessarily other warband mods.
This is because some other mods may enable cross-race units (say immortal landmarks) & mess up the UI tab. The UI tabs can be too small if you have too many units.


Method 2:
My Warband extended mod customized the file for 23 unique categories, and enlarged UI tabs that can fit >70 units per tab (vanilla tab can only fits 20).

Each race has a unique tab associated (BRT, EMP, HEF, etc.). Most tabs still have space for >30 units. You may also re-arrange the vanilla units position to suit your mod.

Then there are two empty category (MOD1, MOD2) dedicated to submod usage. Each category can fit 70 units each, sufficient for almost any mod. If you have modded units of multiple races, they can all fit into these two categories. (So long each unit is unique to one race only, there will be no UI clutter).

To customize background and button for the empty category, you may supply "background_images_mod1.png", "background_images_mod2.png" (and also the button images) in your mod. You will also need to name your mod properly (first letter higher than T), so it loads higher than the main mod.


Method 3:
If you don't like 1 & 2, then you can always reference my modified twui file (e.g., search "mod1" category and track the changes), and create your own UI tabs. There are several things to do (replace GUID, and link GUID properly). So only try this if you are pretty good at xml file.