G'day Merc,
On a single axle trailer make sure opposite wheel is chocked both sides and brake on. Best done attached to the vehicle as the jockey wheel could allow movement when on the jack.
Safety factor.
Place your jack, preferably a hydraulic jack that can take the load, directly under the spring U-bolt fishplate that secures your axle. Loosen the wheel nuts before you lift off the ground. Scissor jacks would not be my choice of jack, but that is me.
Dual axle much the same, but there is less chance of the jockey wheel movement as you can chock the wheels adequately.
Also done on reasonably flat ground is always a good choice. I run my tyres at 50psi, they are light truck, check yours are too. Jim