The fit between the strut pin & the hammer should be a light interference fit, i.e. it should require light taps to insert the strut pin into the hammer. I normally use a 4 ounce ball peen hammer for this job. The fit between the hammer strut pin and the link should be free fitting.
I start by laying the hammer flat on it's side on a bench block or such surface and insert the pin into the hole in the strut, grasping the strut like a handle to use it as a "holder" and to help align the pin it with the hammer hole. If you start the pin straight, you have fought half the battle. If light taps won't start the pin, stop and remove the pin.
If you find the pin to hammer fit is too tight, the easy way to fix it is to remove a little from the pin's OD. The easiest thing for most anyone to do is to hold the pin in the chuck of your electric drill, leaving just a bit more than half the pin's length sticking out of the chuck's jaws. Tear off a strip of 220 grit sandpaper and lay the sandpaper down on the edge of your work bench or other flat surface, rough side up.
Turn on your drill and lay the side of the pin against the sand paper making even contact along the exposed length. You can also use a fine cut file instead of sand paper if you feel comfortable. You can also put a small chamfer on the end of the pin to help you start it into the hole. Check the fit into your hammer as you go and as you feel it getting easier to start into the hole, switch ends of the pin and work the other side. It won't take much sanding before you have removed enough material, so check often.
Be careful of two things - don't beat the end of the pin so hard that you mushroom it. If it's mushroomed now, start with that end exposed in the drill chuck and use a file to remove the mushroom. The other thing? Don't beat on it so hard that you crack or break the side of the hammer! Cracked or broken hammers only happen from abuse and will not be warranted.