I've done plenty of these new (have a whole thread documenting my issues with them following the Ashcroft CV fitment). I use a 3/4" socket set - I use a large socket which fits around the outer part of the seal and the extension. Then with a hammer I tap it in.
As for getting it square I just do it visually.
Tip - put grease on the inside to hold the spring in place. It's not unheard of for this to fall out when the seal is being banged in.
