Think of crumb as a variation on string piecing. Just put all the bits and pieces together to make fabric. You can incorporate blocks/pieces of blocks. You can use a foundation or not.
I don't do this style of thing myself, but I found a person on the board who loves the style and I send her boxes of my crumbs and so I've learned quite a bit about it... for me a crumb is anything smaller than 6.5" square (which is the smallest size I now keep). I keep a flat rate box by my cutting board and in it go all the uneven cut strips (ok, so I'm fussy and prefer straight edges in my storage), the bits and pieces, bad cuts, etc. that happen. I was being pretty productive for a year or two but gosh, it must be 6 months or more since I filled up this last one. A lot of what I send is probably too big for crumbs as direct use, they probably have to be cut a bit. She prefers pieces no smaller than 1.5".
There are some super videos out there.
In a bag of thrift store stuff I got was a bunch (a bag of itself) of precut fan blades, so parallelograms which I would consider crumbs. They are pretty long and I'm going to do a different small piece concept and sew them onto a piece of adding machine tape (then remove the paper). The blade colors are all coordinated and so my bars of fabric should be too, I'm planning on using it then as sashing. I often find bags/boxes of adding machine tape at the thrift store if there is an office supply or craft type section. Here's the thread that gave me that idea:
I don't get what the 'reciept' is for when making string quilts