This post on making WWI aircraft from clothespins is not really a set of instructions, but the photos show clearly enough what has to be done. Brilliant bit of model making.
Biplanes
Fokker World War I Aerial Rules
Fokker is a one-page set of rules for playing miniature games with World War I aircraft.
Fokker Fodder
Fokker Fodder is a set of free wargames rules for World War One aircraft battles. The rules are designed for both single battles, and for campaigns.
Knights of the Sky WWI Aircraft Rules
Knights of the Sky is a set of free wargames rules for WWI Aircraft.