Then I sew another bit to it. At an angle is more interesting than straight. I chain-piece them and chop the threads apart with scissors.
I trim the excess with scissors and chuck it in the bin - there's a bit of wastage but who cares! It's all scraps.
I don't bother ironing at this point - just add another bit of about the right size. If I have a bit that's too long I just cut it off before (or during) sewing. If it's big enough, I put the leftover to one side to start off another block.
I often add pre-sewn bits. The green stripe and brown check triangle was a remnant from some other project. Actually some other project about twelve years ago! Wow.
And another strip - at an angle again. As long as the seam is straight it doesn't matter what relation it has to the edges of the fabric.
Just trim it down with scissors to something that looks normal.
After half a dozen or so bits I flatten it with the iron before adding more.
If there's a really long strip I don't cut it before sewing - just drive it over the top of as many blocks as possible, and cut it apart when you're done.
When it's big enough (I do 6 1/2 inch blocks usually) I iron it, then trim. I do this size block because it's the width of my ruler :)
And ta-da! There's a wee crumb block. All different, but all kind of the same. I don't colour co-ordinate the fabrics at all, although other people do and they look awesome. I try to make sure the edge fabrics have at least a half inch before the seam, to make it easier when I piece them, but I don't always manage it. I usually make about a dozen at a time - chaining them onto each other - then take them away for trimming. It takes about half an hour ... so two dozen for a load of washing or one dozen in the time between putting the roast in and the potatoes ... what an exciting life I lead.