Hi Dario, I can only tell you my method - perhaps others will offer alternatives.
The circlips I am used to tend to be for cars, and they have one end of the circlip turned inward. You can grip the turned-in end in the pliers, which makes inserting them very quick and easy. You can also easily rotate the circlip in its groove, after installing it, by just gripping the turned in bit and rotating the ring toward it, which reduces the ring tension and allows the circlip to slip and rotate. You always rotate the clip in the groove as the final step in installing it, because that verifies that it is fully in the groove all the way around.
With the "value analysed, low cost" design of circlips you have, I would position the clip with its gap adjacent to the cut-away in the circlip groove at the side of the gudgeon pin bore of the piston. Put one end of the clip into the groove, level with the bottom of the cutaway and extending right across the cutaway, while holding the opposite end of the clip in the pliers. Twist the clip moderately with the pliers, which will use its springiness to reduce its diameter so you can start to ease it into the groove. Because you haven't had practice at this, you will need to use something like a piece of brass to help push the clip into the groove, starting near the first end of the clip that you began by putting into the groove, maintaining the twist on the ring with the pliers so that the clip has sprung down to a reduced diameter. Gradually move the piece of brass around the clip as you put more of it into the groove.
When you finish installing the clip, one end of it will run across the cutaway on one side. To remove it, you just grip that part of it, and twist it slightly in the contracting direction while lifting it out of the groove. It is quite easy to do.
