how would I start?

Learn scripting.
Learn how to make AI encounters.
Get tuts.
Mod a map first.
Then make your own BSP.

I have the bsp I have modded several maps and I know how to make ai encounters :P

no i meant SP mod.

anyways, I would start by planning out how the flow of the level will go: big battles, reinforcements, objectives, general path.

Then place all the flags and trigger volumes you will need,as well as actually populating the level with vehicles and scenery (weapons can come later after you've tested the level and see how ti actually plays out.

Next script a bit, until you have a definite idea of what will happen exactly, and place the first few AI encounters. Try creating the level in modules, so you can test certain parts without having to complete the whole thing.

Here's what I did to advance (from your position)

1) Place trigger volumes
2) Script so that something happens when you enter it
3) Make AI encounters
4) Use the ai_place command so that they get placed when you enter the trigger volume
5) Celebrate
6) Place AI all over Battle Creek, and set them to be not initially created
7) Create lots of trigger volumes
8) Kind of make it like a map
9) Use the game_save command a lot
10) Play the map
11) Celebrate

Basically, there are a couple of steps to do:

1) Pre-planning

Perhaps the most vital part. Plan what you want your map to look like. Hover around it in Sapien and look for likely spots that will form into great battles. Plan where you want the player to start, and the areas he/she will move through, and the enemies the player will face, and what weapons can be used to deal with them. A good tip is always to add ammo at points where big battles take place. And health packs too.

2) Adding stuff

Now comes the hard part. You have to add stuff, basically what you planned before. Script. Add triggers. Scripts some more. Add AI. Add objectives. Add cutscenes.

A simple script would look like this:

(script startup teh_script
(sleep_until (volume_test_objects (players))15)
(ai_place )
(game_save_no_timeout)
)

There is a difference between game_save, gam_save_no_timeout and game_save_totally_unsafe.

The classic game_save will check for enemies within a certain radius. If it finds none, it will save. If there are enemies, it will not save. It stops checking after 8 secs. However, the game_save_no_timeout will never stop checking. game_save_totally_unsafe will save the game NO MATTER WHAT, even if the player is losing health in a huge battle.

You have been warned.

Cutscene scripts would look like this:

(cinematic_start)
(cinematic_show_letterbox 1)
(camera_set 200)
(sleep 200)
(cinematic_show_letterbox 0)
(cinematic_stop)
(player_enable_input 1)
(camera_control 0)

You can script up to as many cams as you want.

Objectives are more complicated, ask me if you want an explanation.

3) Test out your map

Compile your level, and test it out! Look for loopholes! Or face the consequences! In my level, http://hce.halomaps.org/index.cfm?fid=4287, I neglected the fact that the player could jump down from the bridge and into the icy area down below, RUNINING the level.

The look for loopholes is extremely important. I had my brother run through the map many times, finding as many loopholes as he could, and giving him a donut for every one he found (anyone have 18 donuts?). Then, I placed trigger volumes with "Guardians" inside to keep you from going that way. However, a few testers found a few bugs with that, so I used a killbox instead.

Heh....managed to jump over your revert box in your Flood Eradication beta2 map. Without cheats.

Why hasn't the topic starter replied yet?