Fully-blown heraldry as we know it developed in the course of the fourteenth and fifteenth centuries.
In the twelfth and thirteenth centuries, nobody should have complex quarterings, labels or obviously later arms. The art of heraldry was a long way off – any knight or lord could make up their own design. However, many of the later heraldic blazons (symbols) had their origins in designs encountered in the Holy Land – for example the Angevin lions and leopards, the German (Roman) eagle, as well as obvious crosses (large or small) and coins (besants – Byzantine currency).
In general twelfth and thirteenth century shields, pennants and surcoats should have relatively simple designs. The background should have one or (at most) two colours, with up to one blazon in a different colour. You may wish to have the same symbol repeated (e.g. three lions), but you should be wary of mixing together too much at once.
Heraldry as we know it was only evolving in the course of the fourteenth century. The celebrated case of Scrope vs Grosvenor (1389) saw two established noble families each claiming the same coat of arms; eventually the Grosvenor family was forced to difference their heraldry.