You will need the 17/19mm spanner from your AM1 tool kit.
Keeping your AM1 upside down, you will note that the chain is still slack.
If you look at the rear wheel you will also note the wheel may be touching one side of the frame as shown.
You will need to straighten the wheel by hand by pulling the entire wheel backwards. This will cause the chain to tension.
Ensure that, as the chain tensions, you keep the rear wheel straight.
Keeping the chain in tension remove the nut covers on both sides of the rear wheel.
Using the included 17/19mm spanner, tighten up the nuts on the axle of the rear wheel, keeping the chain in tension at all times and keep the wheel straight.
Remove the cover over the nut on the side with the motor wire. You only need to expose the nut; you do not have to completely take off this cover.
Tighten up this nut as tightly as you can using the 17mm spanner.
Once you have finished, the chain should look like this. To check the correct tension on the chain, press down on the chain and see that it moves around 5mm. If you over tension the chain it can lead to premature wear of the chainring, sprocket and chain.
The rear wheel should also be central and straight in the frame. If the wheel looks slightly twisted in either direction, loosen off the two axle bolts, re-straighten the wheel and re-tighten.