In some areas of a garment we may want substantial support and we need to figure out a way to create that structure with interfacing. We want that structure to not compress, squish down or lose its shape over time. We don't want that interfacing in the seam allowances though because that will make them thick and bulky.
The choice of structure in a collar for instance, could be something as heavy as buckram or multiple layers of collar canvas ( sized linen ) put together with or without a "glue" to hold the layers together.
Here I have made a rough sample of one layer of linen collar canvas fused (with stitch wichery), to one layer of hair canvas. This may or may not be enough for your purposes, you need to sample different materials to get the right combination.
If you use multiple layers laminated together with a fusible you should stitch through the layers to ensure they will not de-laminate over time.
Cut the laminate to the finished shape and size required - no seam allowances.
Laminate and cotton ready.
Stitch through the layers to hold the laminate together permanently.
Cut a piece of thin cotton larger than the laminate
stitch the laminate to the cotton, stitching through all the layers again.
Trim the cotton leaving a seam allowance beyond the edges of the laminate.
Cut your fashion fabric leaving ample seam allowances
Baste the fashion fabric onto the laminate/cotton. *
* This is important- Baste the fashion fabric to the laminate/cotton over your hand, or a ham or tailor's egg to give it shape. A collar is going to go around the neck, so think of it as a partial circle. The outside of the circle is larger than the inside of the circle so the fashion fabric needs to be slightly bigger. If you just apply it flat on the table, it will resist being forced into a curve and will spend the rest of its days trying to lay flat again, and it will be trying to do that while attached to your jacket!
The fabric thickness should be accounted for as well. Don't forget about turn of cloth! It takes a bit extra ease and seam allowance to wrap a thick fabric around the interfacing to the inside than a thin fabric.
join the laminate to the fashion fabric by stitching about 1/8 inch away from the edge of the laminate through the layers.
Lay it flat on the table- see the slight ripples? You need this ease in the fabric to make a successful collar.
The same technique is used for the cuffs which are actually joined into a circle. Once you sew the cuff seam creating a cylinder you do not want to see any buckling of the structure.