1 - Select a group of mods you want to have work together. Make sure they all have releases related to the same MineCraft version. If you plan to let anyone else use it, also keep an eye on permissions : some mod authors don't allow third-party packs, others will only on request (and then disappear from the internet for months), and some have blanket OKs.
2 - Get a launcher program (FTB_Launcher works for modifying an existing pack, while MultiMC seems best for me for working with rolling your own from scratch), some hard disk space (2+ gb), and time (lots).
3 - If you're rolling from scratch completely, use the Edit Mods option to install the most recent version of MinecraftForge for your version of Minecraft.
4 - Start installing mods, following their individual instructions.
5 - Start Minecraft. Watch Minecraft Crash.
6 - Look at your error log, which will show up as "ForgeModLoader-client-0.log" in your minecraft instance directory. There'll be a lot of stuff here, but the most relevant is almost always a CONFLICT error, resulting from BLOCK ID or ITEM ID overlap. Once you identify the errors and the mods that control them, edit the configs of those mods to use non-overlapping IDs.
7 - Repeat steps 5-6 until your ModLoader log has no more CONFLICT errors. If your minecraft instance now works, success*! If it still crashes, look for SEVERE errors in the log. The related stack trace will often mention which mod caused the crash, and you can try tweaking the mod's config or temporarily removing it.
*Until you run into a bug.