First off, measure and mark where you want your buttonhole to be. I usually only use pins to show where to start and stop, but a tailor's chalk line helps keep things straight.
I am going to assume that your sewing machine has buttonhole settings. This is how mine looks:
Basically four settings: left side, bottom, right side, top.
You do not have to take the fabric out of the machine when using the these presets. Just switch over to the next setting when appropriate. The feed dogs will move your fabric for you. You will also want to set your stitch width to satin stitch (meaning set your machine to make the zigzags of the button hole stitch very close together)
Your start position. Put the needle just to left of your chalked line:
And away you go!
Once you have your buttonhole outline created, then you cut your buttonhole. I use a buttonhole cutter (shown below) but an exacto-blade will also work if you are careful. I do not recommend a regular stitch puller to basically rip a hole in the fabric because it frays things badly.
Once you've got your hole in, rough it up a little to bring up any little frays. Cut off these frays and neaten up the threads.
The second part of the buttonhole is finishing the edges. By far the easiest part. All you want to do is select a zig zag stitch that is slightly wider than your preset buttonhole stitch. (Some experimenting may be necessary here, but you'll only have to figure it out once if you write it down someplace.)
What you are going to do is zig zag over top of the sides of your buttonhole, letting the needle go off the edge of hole (but not onto the otherside - do not close the hole back up!)
This probably isn't a helpful shot, but this is what it looks like:
I'm also going to apologise now, because my machine is acting up and needs to go in for a tune up so it is not as neat as yours will be.
However, this is one side done:
Remember to backstitch the ends.
The otherside: And that's it! Unless you want to use a bit of fray block, just to make sure it won't come loose (good if you are making children's garments and know they will get a lot of abuse from fumbly, learning fingers), but is should be very secure.
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