Hi,
I have 2 maps of my irrigation. One is a general zone overview that was done by taking a top view of the house from the original architect drawings (pdf), marking the heads and connecting all that are on the same zone with a colored marker. I then scanned that so I have it in electronic form.
The other is for cases where I needed to have a more detailed view of the piping, main feeds, and valve box locations. I used the drawing capability of PowerPoint: lines, boxes, notations, etc. I’m very familiar with drawing in this tool so it was an easy choice for me. If you are familiar with a drawing tool that provides these kinds of basic objects, that might be a good place to start.