Note: This recipe makes a ridiculous amount of icing. Below it I will add a note on smaller amounts.
This consistency will be good for outlining. Add a little bit of water to do flooding.
The outline should be dry in 24 hours. Flooding takes longer.
You can store any extra in the fridge. Make sure to cover in cling film (touching the surface of the icing) to avoid it drying out / a crust forming.
For a 10" square cake you can make enough run out to do the top with 1lb of sugar and 2.857 fluid oz of egg whites (roughly 2 medium eggs).
For an even smaller amount (letters for someone's name for example), I'd mix 8oz sugar and 1.428 fluid oz egg whites (roughly 1 medium egg).
Gel based food colouring is best for colouring it. Remember colours tend to dry darker than they are when wet.
Tape your image to a flat surface, then tape wax paper over the top for tracing. Give a colour at least 24 hours of drying before filling in a neighbouring section with a different colour to avoid colour bleeding.
If you are placing it on top of ganache or other "wet" icing you can roll out some roll out icing (Rollout, Rolled Fondant, Regal Ice), let it harden, then put your run out on top and cut around it, then place that on the cake to form a barrier between the run out and the other icing.