It's pretty simple. Just do a make build, or unpack the openbsd base46.tgz, etc46.tgz, and man46.tgz (minimum recommended) into a directory. (Feel free to omit any sets that aren't necessary, but base46.tgz and etc46.tgz are the minimum required.)
Make sure you have a source tree for the same kernel version at /usr/src/sys. If you don't, get a new one.
Then run flashrd against it.
You'll end up with a disk image that you can write to flash. Follow the image installation instructions for further guidance.
flashrd can also write directly to a disk. Whatever geometry the device shows up as will be used for the disklabel this way. Growimg gives you control over geometry, but that shouldn't really matter these days.