You have to keep an open mind, and PUBMED [1] is your friend. Also, if you do not have institutional access to journal articles, Sci-Hub [2] is your friend. Libgen [3] can also be your friend too.
Searching Google Scholar with advanced key terms also helps, but I find it important to keep a paper log of my queries, so I can also follow my thought process.
Ultimately, you have to learn how to be able to "play" with bits and pieces of information that you get from the articles, and make it (relate it) into something meaningful to you. It requires a lot of intuition, and if you find yourself bored, then you are not doing it right. Some people just have this sort of intuition, but anyone honestly can do it. Just focus on never getting bored with the information, and you will eventually learn this skill.
Anyways, the rare disease I have is called autoimmune autonomic ganglionopathy [4]. I wrote a story [5] on how I figured it out awhile back. People seem to enjoy reading it.
I guess the best way to answer your question: I basically searched everything I could about autonomic neuropathy (autoimmune autonomic ganglionopathy is a form of autonomic neuropathy--and I had already been diagnosed with "diabetes-realted autonomic neuropathy"), to see if there was anything that I could possibly relate to my situation. When the situation really got blown out of proportion, I started looking hardcore at really rare stuff.
[1] PUBMED: https://pubmed.ncbi.nlm.nih.gov/
[2] Sci-Hub: https://sci-hub.se/
[3] Libgen: http://libgen.rs/
[4] US Government Information Page on Autoimmune Autonomic Ganglionopathy: https://rarediseases.info.nih.gov/diseases/11917/autoimmune-...
[5] My Story on How I got Diagnosed with Autoimmune Autonomic Ganglionopathy: https://rareandextraordinarycom.wordpress.com/2016/05/14/fir...