Step 1: Go to your tekkit folder in the .technic folder. (Go to the run option (Windows Key + R), and type in %appdata% and find .technic)
Step 2: Download The mod you want for Minecraft 1.2.5
Step 3: Open the folder modpacks/tekkit
Step 4: You'll now have to read the install instructions on the forum you got the mod from. This is because all mods are installed differently on older versions of Minecraft
Ask back if you need help with the mods you pick. I would also just recommend looking it up on YouTube (How to install Millenaire for Minecraft 1.2.5)