You will need:
HMT
the ability to read
Anything but a 56 or, god forbid, 28k connection.
Okay, today, in class "midwaymas's PMI 101" we will learn how to PMI multiple objects into a map, including the actor (another word for ai, and is actr in HMT) actor variant (the different variations of a unit's actors, AKA the weapon they use, and it's called actv in HMT) a biped (character in halo, and is called bipd in HMT) and a vehicle (self explanatory, called vehi in HMT).
The process may, at first, be somewhat tricky. However, with practice, effort, and devotion, PMI'ing will soon become a second nature to you.
The first thing you must do is make these folders on your desktop, or wherever you find convenient.

For those of you who either don't want to or can't load this picture, the folders are:
Build
BSP Build
MP Map Batch Extract
New Items
Alright! Great job, mission one complete! Your almost there! But don't just fell like you've done enough for one night! The fun doesn't stop here!
Mission two! Next item on the to do list is batch extracting the multiplayer map you'll be using, in this case it will be bloodgulch, as all of the greatest are made on bloodgulch.
Open bloodgulch in HMT and go to "Tools->Extract->batch extract"

Select extract metadata and browse to your MP Map Batch Extract folder, the one you just created on your desktop.
When it asks you if you want to extract the BSP, select yes.
In just no time at all, (assuming your computer is only a few months old and isn't crammed with crap) it will say that the batch extraction has been completed! Give your copy of HMT a slap on the back and get back to work!
Now, go back into Tools -> extract, but wait eager mc'beaver! Don't go clicking that batch extract button again! Instead, select the BSP Model sections and save it into your BSP Build folder!

Alright! Great job friend! You sure know how to follow tutorials! Now that you have theBSP model sections saved, close BloodGulch in HMT and open up the SP map you want to use to find the items required.
B40.map will have the majority of the files you will need for more mainstream mods, however, you won't find any flood, monitors, sentinels, captain keyes, marines in their fishing suites, life pods, cryo tubes, and an infinite number of other things.
Anyway, not that you've successfully opened the SP map your going to use, go through and find each item your attempting to PMI into your MP map.

Select the file you are using, check the box that you are saving, and save it into your New Items folder!

Remember, carry out the pmi procedure by pming items in the following order:
biped
actr
actv
This will make it much easier on you. However, don't save your actr or actv into the elite folder, save it into the New Items folder, and it will automatically find it's correct path. If you don't, the actr and actv will make their own sub folders and screw up the PMI process later on.
Now that this is complete, go into tools-> generate raw model offset list (CSV file)
Now go into tools-> extract-> raw models. Load the CSV file and extract each item to it's correct folder.
YOU MUST SAVE IT INTO THE CORRECT FOLDER!
I cannot stress this enough. For example:

This is the correct area to save to. Now into the new items folder, as previously demonstrated. This part of the process only applys to items that have models. This means that your actv and actr do not need to have their raw models extracted because the already have them.
Now that this step is completed, congratulate yourself, for you are only moving farther along the line to the completion of your first (and hopefully not last) successful PMI.
Next, we must travel into the tools->model decrapper/BSP builder area.

Add the raw models you just extracted from you CSVG offset list into the small box shown above. If it tells you that the raw indices or vertices were not found, repeat the step of extracting raw models from your CSV list.

For your index, vertex and sbsp files, return to your BSP build area and select each file as necessary.
Once you have done the above steps, click the button "Decrap these models and build a new BSP chunk while we're at it."
Once it says it has completed the listed task, open up your PMI folder from your desktop.

Copy that file above and paste it into the following folder:
Go into your MP Map batch extract folder, then go to levels ->test ->bloodgulch and paste it their. Say yes when it asks you to overwrite the file.
Now, copy EVERY file in your MP Map batch extract folder and paste it into your build folder.
Once this has been completed, select EVERY folder in your New items folder, and paste it into the build folder.
Now, go into your maps folder, and copy bloodgulch.map from that folder, into your build folder.
Alright! I can see your one dedicated worker! You have managed to stay awake throughout this entire process! Great job!
Now comes the biggest, bestest part of your rebuild! The rebuild!
Close every and all open maps in HMT. Now go to "tools-> rebuild map"

Now , for originol map select the copy of bloodgulch you just saved and click yes to the next question asked.
Next, find each and every item you play to put into your map, including actr, actv, mod2, ipt!, vehi, etc... Copy everything that you remember saving into that additional items folder.
After a short while of waiting, it will ask you if any new verticies have been added to the BSP chunk.
Select yes, and navigate to your BSP build folder, and select bloodgulch.verticies.new

next, select bloodgulch.indicies.new

Now, select yes the the question about offsets.
You are nearly done, but not quite!
Navigate to your build folder through windows explorer and make sure that bloodgulch.rebuild.map is bigger that bloodgulch.map. If it isn't you must now restart!
Copy your rebuilt map into your maps folder, and delete your old one.
Rename your bloodgulch.rebuild.map to just bloodgulch.map
Now, time to go through the last, easy steps.
if you pmi'd any character that can drive a vehicle, then open up HHT and find the vehicle. For the actv, replace the "nulled out" with your pmi'd actv.
Now, there are many people who say you can replace scenery with the wraith. I know for a fact this works. However, this is not the most reliable of things your copy of HMT will ever try and do, so don't blame me if it doesn't work. I strongly recommend you make a backup of your rebuilt map if you attempt this.
here is a short tut on how to:
Select the wraith from your vehicles list.
Look at tag info and look here:

Change that value to this:

Click save and close HMT.
Now, go into sparkedit.
Go to import tag into scenario and select scenery, and import the wraith into the map.
Now good luck, and test your map!
I hope this tutorial worked for you.
Note: Edited recently for spelling errors from way back when.