Some people don't know how to add mods to their clients, so this is for those people:
STEP 1: As always when modding files, BACKUP YOUR WORLDS! You can always redownload the standard Tekkit build, but your worlds can't be replaced that easily without a backup.
STEP 2: Get the mods you want. Currently, Tekkit runs using Minecraft 1.2.5, so get the right version for your mods.
STEP 3: Install the mods in your "modpack.jar" file in your appdata/.techniclauncher/tekkit/bin directory. If you use WinRAR, there will be a dropdown menu on the window that pops up. Make sure you have it ask you before overwriting any files, as they may be needed for Tekkit to run properly,
STEP 4: Run the launcher, but do not login
STEP 5: Go to options and set your used build to "Manual Build Selection". This will prevent the launcher from downloading a normal Tekkit modpack.
Step 6: Test your game on a new world. Seems simple enough, but you need to make sure you haven't damaged your Tekkit installation without risking your world.
If all goes well, you should have the mod fully functional within Tekkit. I've done this with the Advanced HUD mod, and it works flawlessly.
Video walkthrough coming soon.
