Heres an idea that works for me
Cut the required length of thread, estimate the position of the flag on the thread by offering it up to the model. Attatch the flag to the thread prior to attatching it to the model. There is so much rigging already in situ particularly if you are adding the sails that makes it fiddly to add a sticky flag to the thread if it is already attached to the model. Dont worry to much if you dont get the position of the flag quite right because i found you can pull the thread through the flag after it has been attached.
This is the method shown to attatch the flag to the jackstaff (page 8 picture 10)and I found it works just as well for the other flags.
Current Build:-
[color=blue]Stash Avro Lancaster (Full Kit)plus some extra issues I picked up at a good price
[color=blue]Built:- Norman Catapult by Mantua,Roman Catapult by Mantua,HMS Victory,HMS Victory XSection,Focker DB1,Bismark,English Carronade Titanic Lifeboat,American Civil War Gatling,Hummer,Sopwith Camel F1,Occre Jupiter ,OcCre Wagon, Latina Arab Dhow, Eaglemass Orrery OcCre Rogers 119 [color=red][color=red]RB7,OcCre Missippi River Boat, Model Airways Fokker DR1
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