1. The tail goes into the radiator before the valve is attached to the tail.
2. Wrap 15 turns of ordinary (not gas) PTFE onto the threaded part of the tail. If you hold the tail with the thread pointing away from you wrap the tape anti-clockwise onto the thread. If you do it the other way it will unwrap as you screw the tail into the radiator. You can do it with gas PTFE but start with 6 wraps.
3. Screw the tail into the radiator:
3a. If the valve attaches to the tail with a compression nut and olive (as yours sounds like) the tail will have a square or hexagonal part for a spanner just after the threaded bit.
3b. If the valve attaches to the tail via a union connection, there will be an internal hexagon in the tail for an Allen key.
3c. Just occasionally older compression tails have the internal hexagon.
4. The tail should be tight and require a decent pressure on an 8" (length) spanner leaving 1, 2 or 3 threads exposed. If:
4a. It screws right in leaving no threads on show, remove, remove old PTFE, try again with 20 wraps.
4b. It screws in but leaves more than 3 threads on show, remove, remove old PTFE, try again with 10 or 12 wraps.
5. Once the tail is firm in the radiator, slide on the compression nut and olive.
6. Fit the valve, tightening the compression nut onto the threaded valve stem. Tighten until the valve won't slide backwards or forwards on the tail, then another 1/2 turn with a spanner. The valve should not rotate on the tail. Hold the valve upright with a second spanner or grips while tightening. Wrap with cloth or leather to stop the spanner marking the finish.
7. When you have finished and refilled the system, make sure the compression nuts aren't weeping, tightening gently is they are until the weep stops then another 1/4 turn. Don't overtighten the compression nuts on the copper pipes, you can compress the pipe and damage the seal if you do.