How can I make a DIY killer for Johnson grass?

This is a good article that may help https://www.gardenanalyst.com/garden-problems/weeds/control-and-remove-johnson-grass/

Hi Michael, give this recipe a try, It really works for us

https://countrylivinginacariboovalley.com/homemade-poison-ivy-and-weed-killer-that-really-works/

If you are talking about the invasive weedy grass with runners, you really have to use something systemic to kill the roots/runners and tackle it early and often. Pull out as much as you can and amp up the care of your healthy lawn to resist new invasions. If your neighbors have Johnson grass problems you will have to remain vigilant around the perimeter!

Ahhhhhh, Johnson grass!!! There is actually nothing I have found that will organically kill this stuff... for good. Like valleycat1 said, pull as much as you can AS SOON AS you see it. For the rest, I use regular old white vinegar. Just pour it right on. And keep doing it every week or so until it is dead. Remember, tho, that vinegar also kills grass (so be careful when you pour) and birds and wind will bring seeds back into the yard. So one is NEVER free of Johnson grass. When you get it under control well enough, the grass will cover the ground so that only a few seeds will make it to germinate.