METHOD #1: Sit in chair with legs at 45-degree angle and buttocks touching back of chair. Measure from front of knee to back of buttocks and use this measurement to find seat size on size chart.
METHOD #2: Sit in a chair with your shoulders, hips, and heels in a straight line, much as you would when sitting on your horse, making sure that your buttocks don’t touch the chair back. Make a mark directly in front of your crotch and immediately behind your seat and determine that distance to get an approximate seat size.
Continue to sit in the chair and measure the length of your inner thigh from your groin to the middle of your calf with your knees bent. This denotes your upper leg length and the approximate size of a proper saddle flap for your body.
(This method is confusing to me, as they then only discuss using leg length to find seat size).
Sit on a chair with your feet in a parallel position.
Rest your back and buttocks against the back of the chair. Measure the area from the crease of your hip to the end of your kneecap with a cloth measuring tape.
Welcome to the Horseforum! Hope we all can make you feel welcome and share lots of ideas with you.
I've never used any of those. Remember that all the methods and calculators are "estimators". Any result given by any method will give you a starting point. Your real seat size could be anywhere from your measured size to 1/2" in either direction.
Seat sizing will depend on the saddle design: if it has a deep seat, you may feel squished and want a 1/2 size larger - flatter seats give you more room. Some saddles actually measure a little larger or smaller than their specified size.
Seat sizing also depends on your build. If you're really leggy, you may find the perfect seat, but find that your knee comes off the flap and have to go 1/2 size larger to accommodate your leg, etc.
That said, the method I've used is to sit on a chair with my feet flat on the floor, and hips and knees at 90° angle. Measure from the front of the kneecap to where your buttocks hot the chair, in a straight line. This isn't your seat size, but there are tables online that convert this measurement to your approximate seat size. This is very similar to your method 1.
Method 2 looks completely bogus. According to that, I should be using a seat 2-3 inches smaller than what I use.
Method 3 looks like what I do, on;ly you have to use that measurement to find your seat size on a chart or table.
I've also seen calculators where you input various measurements and it approximates for you.
Try 3 different methods and see which seat size show up the most. That is probably a good starting point.
You can also give a description of your build, and a lot of people can get your seat size pretty close based on that.