NOTE: THE INSTALLATION PROCESS HAS CHANGED FOR 1.5.2 - PLEASE FOLLOW THE INSTRUCTIONS AS FOLLOWS IN THE 1.5.2 SECTION BELOW.
Any and all direct Adfly links belong to Flan and/or are taken straight from the mod forum page it comes from.
I claim no ownership over his mod or the adfly rights to any of the links provided. (I don't even have an Adfly account. =P)
I recieve no benefits from the links provided other than the gratification of providing convenience.
So I did a quick search and I couldn't find a comprehensive guide on how to do this. I admit, it's a little confusing with the method when you're using a mod pack but it's quite simple.
Remember when downloading to use the version current to the modpack you intend to install with.
I'll be using Flan's Mod (The main mod) and 4 content packs.
The best way to describe the main mod is it's a module that allows the non-sentient entities into minecraft like vehicles and bullets that are conjured by items such as the vehicle items themselves and the guns.
On it's own, the mod has no content. It's just an api of sorts. Content packs add the items and entities that work with this api.
For the sake of this tutorial, I'll be using 4 content packs listed and linked before. I can confirm these packs work with SMP, though I'll be showing how to install them for both SSP and SMP.
Simple Parts (Required for all content packs listed after this one)
This content pack is contains "useless parts that do nothing". It installs all the basic plane and vehicle components nessesary for crafting without users having to make them themselves, therefore this pack is required for all/most flan content packs.
Ye Olde Pack
This content pack contains the biplane, two seater biplane and a crossbow. This is the most basic of the content packs.
World War II pack
This content pack contains all sorts of planes from World War II including the Spitfire, Fokker and the Lancaster Bomber, panzer tanks, AA guns, jeeps and an arsenal of British, German, American, Russian and Japanese weaponry and uniforms.
Modern Weapons
This content pack contains an arsenal of modern weaponry such as the USP, Barrett 50cal, AK47, Golden Desert eagles, etc as well as Marine camo suits.
SSP (1.4.7)
Download the correct version of the mod and it's content packs and always back up your files.
Open the FTB Launcher and select the modpack you wish to install the mod to. I've found Yogscraft and Ultimate to work fine with this mod. For this example, I'll be using Ultimate.
Press the Edit Modpack button and open the Jar Mods tab and add the main mod's zip file.
For the contents pack, you'll need to get into the folder where the mod pack is installed and into it's minecraft folder. (\FeedTheBeast\Ultimate\minecraft)
In that folder, you'll need to make a folder called "flan" if there isn't one already. As far as I'm aware, the name doesn't have to be case-sensitive.
Extract the content pack zip files. There should be two things inside each one, the folder with the mod's contents and a readme file. Copy the folders inside each zip file into the flan directory you made.
Launch the modpack and enjoy.
SMP (1.4.7)
Remember to back up your files.
Turn off your server.
In your server files, open up your server's jar file with any appropriate program (winrar, etc).
Open up the main mod's zip file with the same program and copy over it's files into your server's jar.
Create a "Flan" directory in the same folder which contains your jar file. (This may be case sensitive.) and doing the same thing as SSP, copy the folders from the content pack's zip files into that directory. You do not need to copy the readme files.
Turn on your server and enjoy. Remember that you need the right version of content packs for both client and server to join.
SSP AND SMP (1.5.2)
Install the most recent Minecraft Forge recommended build and download Flan's Mod 2.3.2 for Minecraft 1.5.2
Open up the FTB Launcher and edit modpack (MODPACK MUST BE 1.5.2!!)
Open the "Mods" tab and add the downloaded .zip.
Run Modpack until at the menu screen, and then close down. (If you need to, open up the texture pack folder via the texture pack menu and go back one folder to access the /minecraft folder)
Open up the directory of the modpack's /minecraft folder and open the /Flan directory. (Folder should exist after running the modpack with Flan's Mod installed. If it doesn't, create it yourself. This may be case sensitive.)
Download content packs from here, then unzip them into their separate folders.
Open each folder and copy the folders inside each zip file into the /flan directory you made.
Run the modpack and enjoy.
Remember that you need the right version of the Mod, Content Packs AND FORGE to make this work!