A Brother Serger is a sewing machine that requires manual threading. You will need to thread the machine if the thread breaks or becomes knotted, or if you've just bought the machine. Threading a Brother Serger requires some familiarity with the movable parts of the machine, so read all the manuals that come with it before you start trying to thread it.
Push the needle base up as high as it will go. Face the machine and find the cover on the front and pull it to the right to pop it open. This will expose a visual guide for threading the loopers. Find the small handle on the right side of the machine and press it up and back to raise the presser foot.
Begin by threading the upper looper, which is color-coded with a green dot. It is the second looper from the right. Pull your thread up through the hole on the thread tree above the machine, then down toward the upper looper. Push the thread sideways into the hook at the top of the upper looper. Press the green tension release disk to the right and guide the thread into the threading channel, which is the crack to the left of the tension release disk.
Find the first hook with the green dot under the visual threading guide. Loop the thread into the hook and pull it to the left, under the second and third hooks with the green dots. Pull the thread toward the back of the Brother Serger and lay it down. Turn the wheel on the left of the machine toward you three turns.
Find the lower looper threader, which is a small metal piece with a red arrow on the end. It is to the right of the needle base. Slide it out as far as it will go. Pull the lower looper thread through its hole on the thread tree. Hook it into the lower looper thread guide. Press the tension disk to the right and guide the thread through the thread channel.
Guide the thread through the four hooks with blue dots, moving it to the left toward the lower looper threader. Locate the square clasp on the threader and hook the thread through it. Push the threader back into place. Grasp the end of the thread with tweezers and pass it through the eye of the lower looper. Pull the thread toward the back of the sewing machine. Turn the hand wheel three times toward you to align the thread.
Thread the right needle thread path next. This is the second cone of thread from the left. Pull the thread up through its hole in the thread tree and hook it into its thread guide. Press the tension disk to the right and pull the thread into the thread channel. Pull the thread to the left and up into the next channel, then over to the left and down. Follow the arrows on the machine that direct you to the correct channels. Grab the end of the thread with tweezers and thread it into the front of the needle on the right. Pull the thread toward the back of the machine.
Thread the left needle thread path. This is the first thread cone on the left. Pull the thread up into its hole in the thread tree, then down through its thread channel while holding the tension disk to the right. Pull the thread through the rest of the channels, which are the same ones you pulled the right needle thread through. Use your tweezers to push the thread through the front of the left needle, then pull the thread toward the back of the machine.