http://www.scoutingresources.org.uk/pioneering_ladders.html - For simple half hitch type and wooden runged sort. If you use wooden rungs, I'd recommend drilling a hole through the planks for security.
There are any number of knots you could use if you are going to make the rungs out of rope. Particularily if you are going to back up the knot with a half-hitch below.
http://www.webstile.ch/knoten/rigging/ch2.pdf - Page 44 for spliced rope rungs.
Depending on the type of rope you have, the splicing can be done different ways. 3 strand laid rope is easy and looks "fancy" with a bit of work. Braided line you might need to figure to a way to make a fid to do the work, but the splices are faster to make.
If it were my ladder, I'd go with spliced rope rungs, but it's up to the user!
The trick with google is to be specific - ["rope ladder" knot instructions] (for example) might work better for you.
Matthias