Setting out timber frame for small extension

[foxtel]

Please help. I am building a small extension to the rear of my home, I have just completed the construction of the sub floor, B and Joist Yellow tongue is in place. My question is setting out the frame. I am almost sure that in a couple of places( the extension is an odd shape) it is not quite square. I know i am meant to flick chalk lines and use that line as a guide for my bottom plate. I want to make sure that the bedrooms etc are square. If i was to just follow my sub floor i know that this will not give the desired result. So in otherwords I know that the frame may slightly overhang the yellow tongue flooring for example in some places. HOW DO I FLICK THE CHALK LINES TO MAKE SURE ALL THE ROOMS ARE STRAIGHT AND SQUARE. I am very new to this first time I have built anything.

Thanks In Advance.

[Tools]

The best thing is to post a pic of your plans so we can see what you are trying to achieve.

Tools

[Master Splinter]

To get 90 degree angles, use the 3-4-5 triangle approach. Measure and mark 300mm along one chalkline, 400mm along the other that is supposed to be at right angles, and then measure between the two points you just marked. Any measure other than 500mm means it's not a right angle. Scale up to 600-800-1000mm if you want more precision. (or 3 meters - 4 meters - 5 meters if you want to be silly!)

See here for a nice illustration.

[Metung]

If you use the 3-4-5 triangle approach, make sure you then measure the diagonals (corner to opposite corner) to ensure they are the same. If not, then your room isn't square.