From Lifehacker/Jalopnik:

https://truckyeah.jalopnik.com/how-to-avoid-the-perils-of-an-unbalanced-trailer-1519952988/@whitsongordon

The basics:

• Load 60% of the total weight in the front half of the trailer.
• The axle isn’t always at the middle of the trailer.
• You’re looking for your hitch ball and the hitch to drop right at one inch to indicate proper trailer weight, tongue weight, and weight distribution.
• Like in snow — if you get into a problem, DON’T TOUCH THE BRAKE!

The article explains why.