Before creating a server for an existing client modpack it is vitally important that:
If you are using Forge <=1.6.4 all ID conflicts for both items and blocks must be resolved. Most block ID conflicts will crash the client or produce an IDConflicts.txt file, but not always. Most item ID conflicts will not crash the client (at least not initially). Even if the client pack starts successfuly you need to search the ForgeModLoader-client-0.log when using a <=1.6.4 modpack for the word CONFLICT. Not all conflicts can be resolved (Mystcraft, many slabs, some Gravestone stuff) but most of them need to be addressed. Specifically focus on conflicts that list two completely different mods in the conflict.
The client must NOT have an automated ID fixer installed. All conflicts need to be resolved in the configs. It is safe to use IDFixMinus as long as it is removed after all the conflicts have been resolved.
The client modpack MUST have a full compliment of config files. These files get created the first time mods get injected into Forge. Even with modpacks for Forge >=1.7.2 the configs need to be included to ensure that they are synchronized between the client and server.
I recommend the use of Cauldron over basic Forge for running a server. Cauldron has Spigot built in along with other optimizations which make things run better (usually). Cauldron also integrates the Bukkit API so you can use Bukkit plugins if you choose. Cauldron is no longer available through legitimate means. If you are creating a new server for Forge <= 1.7.10 you will need to use vanilla Forge.
This guide assumes you are using Java 8. If you are building a server for 1.6.4 or 1.7.2 you will need Lexmanos' Legacy Java Fixer mod. If you are building a server for 1.7.10 you will need to use Forge #1230 or later.
Download the latest version of the Cauldron Forge installer for your version of MC: http://files.minecraftforge.net
Install Cauldron Forge to a new folder. We'll refer to that folder as SERVER.
Copy your config and mods folders from your client pack to SERVER. If you have other mods which require top level folders (like BetterDungeons or Flan for example) those must be copied over as well.
Remove any client-only mods from SERVERmods. These are mods which only operate on the client. Mods like Animated Player, ArmorStatusHUD, Damage Indicators, Minimap mods, NotEnoughKeys, etc. Basically if the mod only changes the way that things look in the game, but not saved into the world it is probably a client only mod. Most mod authors are kind enough to indicate when their mods are client-only. If you start the pack and are getting errors about "client" or "render" then you probably have a client-only mod in your pack.
Create a batch or script file to start your server. I won't get into the intricacies of fancy Java parameters here. This is a simple way to start your server:
startserver.bat:
@echo off
java -Xms2048m -Xmx2048m -jar forge-server-filename.jar nogui
startserver.sh:
#!/bin/bash
java -Xms2048m -Xmx2048m -jar forge-server-filename.jar nogui
Adjust the -Xms and -Xmx parameters to suit your needs and client load.
Change "forge-server-filename.jar" to the actual filename for your Forge JAR.
If running on a Linux server I highly recommend the use of Linux screen. Google for detailed usage instructions.
Make sure you have the latest version of Java 8 for your OS. I recommend using the JDK instead of the JRE because it comes with some debugging tools which can be useful in the even of crashes and hangs.
Make sure you have some kind of backup solution in place. Use a mod or some external solution, but use something lest you end up crying in your Wheaties.
