Identify the parts of your tape recorder. Though designs vary by machine and manufacturer, the same components will be found on each. The supply spindle is on the left. feeding tape across the heads to the take-up spindle on the right. There is a variety of rollers to ensure supply of tape and constant speed across the tape heads. This is where you will find the biggest variation from machine to machine.
Place a full reel on the supply-side spindle. Tape will unwind from the top on the left side of the reel. Place the empty reel on the take-up spindle on the right side. No motors should be running at this point. Some recorders require power to thread tape; others do not.
Identify the tape path. On the supply side, there is often a tension roller below and to the right of the supply spindle. Tape will thread over this roller and back toward the left. There may be one or more guide rollers before the tape gets to the head assembly. Tape rests against the guide rollers (no threading between them is needed) but must pass between the capstan and pinch roller, usually on the right side of the head assembly. Both reels will operate in a counterclockwise rotation.
Thread the take-up reel. There may or may not be additional guide rollers before the take-up reel. Tape goes around the reel from the right side. At the hub of the take-up reel, there will be one or more slots for the end of the tape to pass through. Feed about three inches of tape through the slot (it will stick out above the reel) and rotate the reel counterclockwise to wrap the free end several times and to take up slack from the supply side. The recorder is now ready to operate.