For me the easiest is to start with the 4 circles with the parallel lines from the above post.
The more detailed, the more precise the astigmatism fans / circles are, the easier it is to get lost with them, i.e. be unsure about which line has the most blur.
With the 4 circles I go as close as needed to get the lines black and equally sharp on all 4 directions. Then I start increasing the distance and notice which one goes grey or blurry first.
Because cyl can be different per eye, I’d recommend measuring it per eye.
This basic test will give you an idea if you really have an astigmatism (transient or permanent) issue at the moment. But note that astigmatism can change in power and in axis quite quickly especially if you are not wearing corrections for the cyl. I could get 5 different measurement from 5 different optos within 5 days. Different in axis and different in strength…
I most typically have cyl prescribed by optos around 90 degrees (anything from 86 to 104 degrees, and 0 to -0.75D in strength), and not surprisingly the first circle that blurs is the one with the vertical lines. Because I’m just checking the presence of astigmatism but I do not add to corrections, it is enough for me to know it is around 90 degrees and if it is significant or not on the given day.
But I could then go on looking at the more detailed drawings, do the same process to figure out if it is 85, 90 , 95, 100 or 105. Then I could go further with test lens kit or other tools to guess what in between the 5 degrees exactly.
20 years ago, most optos corrected to rounded 5 or 10 degrees, nowadays we can measure so well that optos prescribe cyl by 1 degree. But all optos I talk to admit that the error is minimum +/- 5 degrees, but sometimes it is +/- 15 degrees, because the “which is better this or this?” question is very subjective.
20 years ago most optos - at least the ones I met - started to convert cyl to sph automatically if cyl was measured above -0.5D to keep the prescriptions with low complexity and to allow for contact lenses without cyl correction (because the ones with cyl added rotate constantly on the eye making them super uncomfortable)
Whichever is the direction of lines that blurs first, that will be the axis corrected with cyl if you let the opto do that.
This picture shows what you should see before you start increasing the distance (right side) and how the blur appears first (left side). So in the example the blur appears around 10-11 or 4-5
Looking at a test lens frame that 10-11 / 4-5 will translate to a cyl correction at about 120 to 150 degrees.
Note: The cyl correction in the glasses is stronger diopter added on one line like this (obviously then worked together with the main diopter so the edges are not as sharp as on this pic but gradually changes from the cyl extra to the main)
So the cyl is one line but your eyes can have uneven surface and have a main angle and a secondary angle. That’s why in my opinion 90% of cyl corrections is almost pseudo scientific… All depends on how you are that day and you might unconsciously tweak the result by turning your head or eyes left and right or up and down by just a few mms…