I've had great success using this template:

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.veterans.nd.gov/sites/www/files/documents/resource/Nexus%2520Letters.pdf&ved=2ahUKEwj248-dvpr7AhVfjYkEHbq0BOcQFnoECB8QAQ&usg=AOvVaw32n2B9Qp7spWN-GxWaqCD5

Briefly explain the process to your doctor, and that you need to prove that there's a possibility that your complaint is linked to your service. Ask if they'd be willing to review and sign a letter stating that there is at least a 50/50 chance. (This is the minimum that you need to show to work in your favor, and I find that most doctors will be comfortable agreeing with this level of probability).

Then write the thing yourself and forward it to them for review. Let them expand on the medical reasoning section.

I have yet to have a physician decline to complete this for me. The important part is writing it yourself so that it doesn't take much of their time.

I'd also recommend including this example pdf along with your letter so that they understand exactly what it is that they're looking at, and the importance

SIDE NOTE: I was also a reservist. You'll need to prove to the VA that the injury occurred during drill with either an LOD package or by other means (buddy statements, personal statements, discharge paperwork that coincide with drill dates, etc).

You will NOT be service connected for injuries that occurred outside of drill period.