Since I portray a Flemish ally of the English, my first stop in devising my shield was a look at the flags and banners of old Flanders. I found a wealth of information on the Flags of the World site, which I cannot recommend highly enough. Most of the following information comes from there.
The historic flag of Flanders, still used by historical re-enactors and the occasional separatist, looks like this.
The rampant lion is derived from the earlier Duchy of Brabant, and still earlier from the seal of Philip of Alsace, dating back to 1162 AD.
The city of Ghent showed the lion in white (argent) like this:
In fact, it seems that everything Flemish has some variant of the rampant lion. This flag represented the powerful Guild of Weavers in Ghent, and combines the lions of Flanders and Ghent with a blue lion for their guild.
It was natural for me to use some version of the lion, but of course I couldn't have any actual Flemish flags, much less the seal of modern Belgium. I put the lion on a white background and added a chief with three crosses to make sure it was distinctly mine.
In war, the appropriate battle cry for my character would be "Vlaenderen die Leeu" (Flanders the lion).
This motto was present on the arms of Pieter de Coninck at the Battle of the Golden Spurs on July 11, 1302. When it was clear that the French side was losing to the rebellious Flemings, some 300 Leliaarts (Flemish vassals of the French) took up the cry and tried to escape by mingling with the Flemish partisans. But, they were wearing riding spurs, unlike the Flemings who fought on foot. Guy of Namur ordered them all killed, and the battle is named for their telltale spurs.