Part of the good news with surcoats is that they don't have to match your shield/heraldry. They can, but don't have to. You can work all, or just some elements into it, rearrange the charges, or whatever. It's all cool, from a historical standpoint.
But something you might want to consider is whether you want to create your own heraldry, and whether or not you want to design a surcoat to that. Another idea is to go with the templar motif, and then work the cross into heraldry later. Or not, it's all up to you.
The charges can be done in a variety of ways, from appliques (sewing on a cut-out piece of fabric in the right shape), to embroidery, to painting it on with fabric paints and a stencil. The one in my avatar image currently has the charges done with appliques, and a "button-hole" sort of thick stitch all the way around. I had that one custom made for me by Jess Finley at
Fuhlen Designs. A better view of it (photos are clicky or bigger view):
My previous surcoats were all made by my own hand. The large "ankh" charge on this one is an applique too, but with a simple line-stitch (with the fabric folded in at the edge) all the way around:
I didn't use an actual pattern for mine, but rather eyeballed it. The important things to remember when making a surcoat, IMHO:
1. It should come in at the shoulders, rather than just be a rectangle with arm-holes, otherwise the shoulders stick out way too far.
2. It needs to flare out at the bottom. That is, the "skirt" section needs to widen toward the bottom, like an actual skirt. If it doesn't do this, and is just a rectangle, it will look ridiculously too tight around the legs. The ones I made for myself have a subtle widening here, but the one made by Jess is a huge skirt.