Repairing a crazy quilt with fabric or patches harvested from another portion of the quilt is usually done when the quilt is being downsized from its original dimensions. Over time, the edges of a quilt can wear to the point of becoming shredded while the center remains intact, in which case the renovator will "cut down" the quilt by trimming away the worn edges and squaring it off to a smaller size. Some usable fabric can generally be salvaged from the trimmings, and used for repairs.
When you're examining trimmings for harvest material, keep in mind that any frayed, worn or otherwise fragile fabric is not going to last. You need a strong, intact, well-preserved patch that isn't likely to disintegrate in a few years; if you can't find that it's better to save yourself the headaches of having to redo the patch in the future and just use lace or new fabric.
Because the satin backing material for this piece is heavily soiled and snagged, and yet still has some pretty quilted areas that are usable, I'm going to harvest a repair patch from the backing.
Here's one piece I cut from the back after I separated it from the silk patchwork top:
This one I'm going to save for another project, because it's too big and in too good condition to cut up for patches. But here's another section of the backing that was spoiled by whoever cut it up:
From this piece I cut and fit* a patch over a torn silk patch of the same color (Btw, I don't recommend using old satin for patchwork if you're not an experienced sewer because it's pretty slippery and hard to fit and stitch into place. Careful as I was, my patch puckered a bit on me, which I'll deal with later on when I embellish.) One fabric I never use is old silk; it becomes brittle and dry with age and it's impossible to get clean. Also, most old red silk dyes are not colorfast and will bleed if washed, making any piece you repair with them dry-clean only.
Also, remember to gently hand-wash any harvest material, air dry it and press it before you use it for repairs. You can really see how soiled the satin is when you compare the color of the washed/finished patch to the original. The material is so dirty it turned the water in my hand basin brown.
Whenever you cut down an old quilt (and you have the storage space) try to save what material you can from the trimmings. Very often I find old feedsacks and calico that I save from one quilt will work better than new fabric as repair patches on another quilt made in the same time period.
*If you've never tried fitting a repair patch, I posted a tutorial with photos on how I do it over on the stories blog here.