Not hard at all... just have 2 drift pins available (they are different sizes) to replace the 2 pins as you work them out .. Then mate the old sear spring retainer pin to the new side plate assembly and work the 2 pins back into the pistol while allowing the drift pins to exit back out the other side.
If you don't use the replacement pins to hold the parts (hammer and sear spring and a couple others) in place, they will be falling out on your table causing you more time to put the thing back together...and of course, frustration...
Good luck!!