Like SouthernTrails said, those are far too narrow (the problem with that is that they are more likely to break, especially since they are old, and that would be very
traumatic for a young horse) and I suspect they are far too short as well. The one with the buckle actually looks like it might be a stirrup leather off of an English saddle.
It doesn't take a whole lot of money to get a decent latigo (either nylon or leather) that will last for years. Since you're not riding anyway, why not just wait until you can afford something that's actually supposed
to be used that way instead of risking traumatizing your young horse?
AFTER you get some real latigos, it's not all that complicated to attach them to the saddle. You just tie the end that's doubled over around the front D-rings of the saddle like this How To Attach a Latigo (cinch strap) to a Saddle
Then you proceed to do up the cinch in one of the ways from the link that STGA posted earlier.