Stake and String
Drive a wooden or metal stake into the ground with a hammer at the location where you want your wall to start.
Drive a second wooden or metal stake into the ground with a hammer at the location where you want your wall to end.
Tie a string to the first stake.
Slide a line level onto the string if you need the line to indicate level as well as maintain straightness. If you do not require a level string, you can forgo this.
Tie the other end of the string to the second stake in a manner that forces the string taut.
Measure outward from the exterior wall with a tape measure and scribe a line on the floor that marks the location where you need the front of your wall studs to position. Place the first mark at the far right of the wall and then place a second mark at the far left of the wall.
Place a string into a chalk box or utilize a chalk snap string that unwinds from a plastic housing and already has chalk on it. If you use a chalk snap string, unwind enough string to extend between both marks on the wall.
Hold the string against the first mark you placed on the floor. Have a second person extend the string down the length of the exterior wall and hold the other end of the string against the second mark. Hold the string taut.
Reach towards the middle of the string, pull it up from the floor and allow it to snap back against the floor. Repeat this two or three times.
Lift the string up to expose the chalk line on the floor.