Decks, Patios, Porches and Docks - Getting square

Yes I know this is deck forum but my question is about a playhouse and I know when you build a deck you have to do this everyday. I want to build my kids a playhouse I am going to do it by putting 8 foot 4by4's as corners and two 10 foot one's in the middle of those for the center rafter. My question is what is the best way to get them in a perfect square and level and plum. I am pretty handy and will not have any trouble after this but I know if you don't get it centered and square it won't be worth building. Thanks for any help Steve

As far as level and plumb, use a level. For square, the simplest way I've found is to measure the diagonals of the box - if they're equal, the box is square. If not, adjust accordingly.

Make a stringline boxfor the outside dimension of the posts. Use the 3' 4' 5' or 6' 8' 10' or the diagonal method to check for square.

Do you remember the Pythagorem theorem from geometry class? A sq + B sq = C sq

Plum is found with a GOOD level