In this tutorial I'll be showing you how to add the Mongoose.
Download here: http://hce.halomaps.org/index.cfm?pg=3&fid=1539
Well to start off make sure that your directory is set up correctly...should look like this (or similar if you are using another vehicle):

Now, please note that if you later get a "whoops" error when you add the tag in sapien (happens with any tags, not just vehicles), it means that your tag files arn't in the right spot. I'll show how to fix this later in the tutorial.
Now open up geurilla. Open up the globals.globals file in your tags/globals folder. Scroll down to where I am in this picture to the vehicles section.

Okay...click the add button and then click the "..." to add your ".vehicle" tag. When you add the vehicle it should look like this...

Now close the file (save it of course

Do whatever you need to do first (you know radiosity and such...I like to do it in the command prompt though because it works better for me.) and then click "edit types" in the hierarchy view. Add the vehicle from your vehicle's tag folder and click done/ok or whatever.

Now go to your vehicle section under the hierarchy view and go up the properties view and scroll to your new vehicle in the "types" section.
Right click somewhere on your map and look at your awesome new ride


Okay, notice I circled a section in the properties pallet about spawn flags.
Make SURE that all of these that I check you check! (unless you only want it for a certain gametype...then only check like the slayer ones or whatever) This pretty much tells halo that you want your vehicle to be allowed to spawn in all game types. It will not spawn otherwise.
Do whatever else you want to your map (adding scenery, spawn points, the goods


Well that's it! If you followed these steps correctly you should have a new vehicle in your map without replacing any others! And it's sooo easy to do isn't it?

ABOUT THE WHOOPS ERROR: *gasp*
I tried to add my new tag and it gave me an error! WHAT TO DO?!?!
Well this is an easy fix, but I don't know how well I can explain it...
First what you want to do is try to add your tag. When you get the whoops error delete the bad tag from the list and press done, and ok to exit the tag adding menu. After you exit this menu it should temporarily show a list of errors that sapien received after you tried to load the tag. Press the "printscreen" button quickly before it disappears and paste it in paint.
While you have the image up, look at the names of tags it couldn't find.
I'll use one of my tags as an example...

As you can see my pelican tag has some shaders, effects, etc. it can't find.
This simply means that some files aren't pointing to the right place.
IF YOU EVER GET THE WHOOPS ERROR IN SAPIEN DO NOT SAVE CHANGES TO YOUR MAP. THE NEXT TIME YOU TRY TO OPEN IT, SAPIEN WILL SUDDENLY CLOSE AND CRASH WHEN YOU OPEN YOUR MAP.
Open up geurilla.

Once the tags point to the right place and there are NO files in red in any tag for your vehicle, open up sapien again and try to add your vehicle. If the problem persists, repeat the above steps for the missing files it lists. (take a screenshot of the errors it gives you when you add it).
Well I will go finish up on fixing my pelican. The same rules apply for all tags: shaders, effects, sounds, etc. Just find which tag needs them and change the path so that it is pointing to the right directory.
I guess this turned out longer than I expected, but I hope this tutorial helped you all.
EDIT: Just had quite a shock lol
I finished fixing my pelican's sound tag errors and added it to my map. I added a "NONE" tag by mistake to my map and then switched it out for the pelican. My computer crashed and my keyboard temporarily broke! I went back into sapien to make sure my map still worked...it did. And I tried adding the pelican again which this time it worked 100%. Silly sapien!
Just be aware that maybe sapien doesn't like it when you swap a custom tag from a NONE tag.