He started with the Romer scale (brine freezes at zero, water 7.5, boils at 60, body temperature 22.5), which he tweaked to not need fractions for plain water freezing and body temperature by fudging some numbers and multiplying by four.
This made water freeze at 30 and human body temperature 90. He recalibrated it so that it was 32 and 96 so that there were 64 degrees between them, so he could draw the markings by dividing the interval between them in half six times.
He then saw that water boiled at about 212 on this scale, so he tweaked it again so that water froze at 32 and boiled at 212, since they’re 180 degrees apart, which is desirable because it puts them on opposite sides of a temperature gauge.
Because of these tweaks, the original brine temperature is now about 4F, and body temperature is 98.6.
The tweaks make sense if you know that Fahrenheit was making and selling temperature gauges, so taking the Romer scale and marking every quarter degree gets you the first Fahrenheit scale.
Then he tweaked it to make it easier to produce, and then again to fit in the dial better.