Off the top of my head Beryl Dean brought out a book on making church embroideries, although it was written in the 60's it is very good for making up detail. I know I've come across other books on banner making, but it is such a while that I don't remember their names or authors.
For banner making for a church designs need to be bold enough to be seen from a long distance but also (I think) want interesting detail from close to, added to which colours change from a distance - you will need to take this into consideration when auditioning fabris(and get fit from walking from one end of the church to another and back again to check ).
Another consideration it the actual building - does it get damp? if so make sure that all the fabrics are washed and pre-shrunk.
Check out about how they will be hung, ask the priest/churchwardens about any special requiements or restrictions for putting them up (I am in the Church of England and nearly all of our churches are 'listed buildings' and as such we need special permissions to make certain permanant alterations to the building)
Apart from that just follow the usual guidelines for making a wall hanging. Good luck and I will be interested to see what you come up with.
Embroideress Extrordinaire & Mad Hatter
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