When working out an approach prior to the shoot, this type of diagram helps you to make a list of exactly what type and how many instruments you need for the job. It's sort of like a visual chck list for your shoot. Also, it's a good idea when making a lighting call for a shoot to always order a little extra of everything, lighting fixtures included. Some stages or locations have no supplies, so if you don't ask for it on the truck, it's not going to be there. Make the diagram as big as possible on the paper you're using.
Small diagrams get very cramped and are difficult to make notes on.
Use directional arrows when the purpose of the light might be confused. Don't be afraid to make notes for all the particulars with each light. It's the little things that make the difference in lighting. Professional templates are available at fine art stores, but those are rarely necessary. If you or your client can understand your own drawings, you're doing great.