Easy spoke length guide for new motor (rear) and rim build?

[alsmith]

Firstly- I've never built a wheel before, the closest I've got is spoke replacement.
There have been a couple of good guides to the wheel building linked to on the forum so that should be ok (apart from time and maybe a re-do because of something stupid).
What is the easiest (detailed) guide for measuring spoke lengths required for a rear wheel motor with a generic rim? Offset will be needed. I don't have the motor yet, I have a suitable double walled rim (not measured yet) but want to get the measuring method clear in my head before I buy the spokes.

 

[wesnewell]

These will give you all the info you need.
https://www.google.com/search?q=effective+rim+diameter
http://leonard.io/edd/howtomeasure

 

[alsmith]

Many thanks. I'd tried searching but everthing just assumed you had the spokes or glossed over it.