Insert the splines into the new insert. You may find use of a gentle clip to hold them in place will help.

I slide the spline all the way to the end of the groove at the "Tip" end of the insert. This gives the insert some rigidity (stop giggling, boys) for initial insertion into the blade's brackets.

Insert the insert with splines into the blade groove, starting with the "Tip" end of the insert. Slide it into the "soft edge" end of the blade. Gently work the insert into the blade and move/remove the clips as you proceed.

I find that this is easiest way to do this is with the blade held vertically, sliding the insert in upwards while hanging down to the ground. This keeps the splines hanging down with the insert, but the clips are really helpful to keep the splines from just falling or sliding out of the insert.