The fixed gear single-speed bike (AKA"Fixie") requires correct and taut drive chain tension. (nothing worse than a "sloppy" chain)
This is never a problem on dérailleur gear bikes as chain tension is constantly regulated by a rotating, spring loaded arm which carries the jockey and idler wheels over which the chain runs.
With the fixed gear bike, I find it difficult to single-handedly pull up on chain tension and then tighten the axle nuts.
Therefore I have devised a method using a short length of rope, 2 guide bolts and the rear wheel itself to tension up the chain whilst you nip-up the axle nuts.

Thread the rope through the seat stay crossmember as shown. (this I call the "fixed-end")
Loosen axle nuts sufficiently to give free axle movement in the dropouts.

On both sides screw a suitable bolt into the mudguard stay fixing lugs, so that it protrudes through, stopping just short of the hub (wheel must be free to rotate).
I call these guide bolts.
On each side; thread the rope under the axle and over the guide bolts.

The 2 "free ends" of the rope are then gathered up and tied around the rim.

By grabbing the wheel rim/rope and rotating the wheel backwards the rope will become tight and draw-up the axle into the dropouts; thereby tensioning the chain.
Ensure the wheel is aligned midway b/w the chainstays and with your other free hand; tighten up the fixing nuts.
The rope and the guide bolts are then removed, and with a final check on wheel nut tightness - the job is done!