If you can not manage it with a pair of needle nosed pliers then go to a hobby shop and buy some tube that will just fit inside the gudgeon hole.
Then something solid that is a good fit inside the tube
Put the clip into one end of the tube.
Put the tube into the pin hole.
Push it out with the piece of solid rod
Use a screwdriver or pick to slide the clip into the groove.
Helps if you do the whole thing inside a box to catch the clip as it goes flying past.
If u have an old clip like I've see then there is no "tab" and is alot more touchy but if u have a new clip then....
Assuming u are familiar with the groove the clip fits in,
- take the end of the clip without the tab and wedge it in the groove. my pic doesnt have it but some stihl pistons have a cut at the bottom so the hole for the wrist pin is not a full circle. if so, put the end inthe opening at the bottom.
-take the needle nose pliers and grab the tab.
-start pushing the tab to compress the clip while gently pushing the clip into the opening with your finger\another tool until it slides into the hole. slide\tap it in until the whole clip falls into the groove. Thats it!