How do you relate other medical conditions to service related conditions?

The VA has publically available letters and instructions that lay out relationships between maladies. They might be a little more interesting than a supply manual, but I won't bet on it. I was able to find them using google and a lot of perseverance. Once you find them, look up you particular malady and you'll find paragraphs below the initial discussion of why it isn't allowed where it reverses itself and says it is allowed under these conditions. Good luck, it's a pain, but it's there.

So I have been down this road a couple of times and I think you are talking about secondary conditions? What you must show Is how the service connected condition Ie: SC for PTSD and it agrivates my Sleep Apnea. You will need a nexus letter to connect the dots. You will also want an independent review of how your condition aggrieves your non service connected condition.

LTC Troy Williams my father in law (Vietnam era vet) has just had his primary care doctor or the diagnosing specialist write a letter that links it in their medical opinion. You file that with the claim. He's had all kinds of stuff picked up by the VA.

I have an Uncle that has had a number of issues connected to Agent Orange exposure and a rare liver disease and degenerative eye condition that has three sources, all of which he was exposed to in Vietnam. He's had to go back for subsequent claims. Same way.

I would go get assistance from a Veteran Service Organization, like DAV to help you. This is what they do. https://www.dav.org/learn-more/news/2017/take-control-claim/