Here is my view and my limited knowledge on ferning. I learned of it last month and researched like a crazy person about everything fertility. Now I am a bit more sane and relaxed.
Ferning is a phenomenon that occurs in our fluids i.e. cervical and salivary fluids. An abstract can be found here
https://www.craigmedical.com/ovulation_abstract.htm
It is believed that the high increase in estrogen that is released right before a woman ovulates causes these ferns to occur in her dried saliva. (TMI: I tried looking at the Cervical mucus under the microscope, I never saw any ferning whatsoever, so stick with the saliva if you are going to try this.)
There are many websites that try to show what no ferning is vs partial ferning vs full ferning. Those pictures are a very very very basic starting point, and can lead to frustration, disheartenment, and outrage. Generally no ferning, has no ferns or what I call bubble ferns. Partial ferning can be anything between scratchy looking ferns to wavy small ferns. Full ferning is long straight ferns that have no bubbles and are really close together.
I (b/c I am a nerd), conducted my own experimental observations so that I could better understand my body. I bought a microscope off of Amazon and bought 100 slides. Every morning when I rose, I "gleeted" on my slide. (That is when you cause a steam of saliva to flow out like a sprinkler from under your tongue. I am such a tomboy.) Gleeting as opposed to spitting on the slide allowed me to attain a perfect sample void of useless bubbles. It was the act of saving these slides that allowed me to get an idea of how No/partial/and full ferning looked for MY OWN BODY. Once you have an idea, you do not need all of the equipment.
If you are at all interested in looking at ferning, I would advise you to go the children's section and getting a toy microscope for $10.00. It works and allows you to use slides, unlike the lipstick versions. The lipstick version magnifies the saliva sample 60X, most toy microscopes magnify the sample 100X. I figured, I can give the microscope to my baby when they get 8 years old....lol
After my experimental fertility month I learned that all of my signs lined up perfectly. That meant there was no need to use a million methods when one or two would suffice. OPKs, Ferning, CP, CM, and BBT. So I chose the Cervical mucus and BBT this month. I only OPK'd when I saw EWCM, it of course confirm what I already knew. BBT allows me to see when I have ovulated (which gives me a way more accurate delivery date.)
Hope all of this is helpful.