Another example is Heather Hopp-Bruce’s “DIY Canvas” published by The Boston Globe as part of its efforts to encourage its readers to come up with new designs for Boston’s flag:
At 24 pages it’s much longer than a worksheet, but another nice resource is Jonathan Parson’s Flag Toolkit designed as a resource for primary school teachers participating in the 2015 Flag Project of the UK Parliament.
Last but not least, the most popular general resource for good flag design is Good Flag, Bad Flag (compiled by the PFA’s Ted Kaye), a 16-page booklet published by the North American Vexillological Association in 2006. Amazon is currently selling it for $2.99, or you can download the PDF for free. (PDF versions are also available in five other languages besides English.)