When trying out the shoulder guard, the most important thing is that the shoulder straps are centered on the shoulders. It should also have reinforcements at the collarbone, chest and spine. Most models have adjustable straps to customize the protector so that it fits properly on the body.

An important piece of protection, especially at younger ages. The elbow protector is one of the easiest protectors to try out. It is important that the elbow is in the center of the guard's scald, then tighten the straps so that the guard cannot spin.
It is also good if the forearm protector covers all the way to the glove. This will minimize the risk of injury.

You should make sure that there is no gap between the glove and the elbow pad, as this part of the body is exposed to blows and impacts. It is up to you whether you want the glove to be loose or tight, but it should not be so loose that it loses its shape or it will not provide the required protection.
Fingers should not reach all the way into the glove. This is to avoid the fingertips hitting the ice/snow.

The main function of trousers is to protect the hips and the front of the thigh. Don't buy pants that are too big, as they can restrict movement. The easiest way is to start with your waist size. The trousers should sit well on the hip bone. The length should be such that the trousers extend 3 cm above the shin guard when walking upright, then the trousers are sufficient even when sitting deeper in the riding position.

The kneecap should fit in the center of the kneecap and then provide good protection all the way down to the ankle. Make sure the protector is not too long - then the skates will push the protector up and the kneecap will be out of position. Also be sure to fasten the protector properly with shin guards (or tape).
A good tip: Place the shin guard in front of the boot to better protect the tendons in the ankle.