Someone told me to order precut stud and then add a 1 foot header above the sidewalls. how does that work?
Please take this the right way....but if you are asking these kind of questions related to 'pre-cut" and 'headers', it would appear that you don't have a solid set of plans and that maybe you are potentially a little in over your head.
Thre is a bit more involved than just nailing up some pre-cut 2x4's....
Sill plate needs to be pressure treated wood.
Anchor bolts in the right locations
Depending on building codes, holddowns as well
Shear walls
If the attic is going to be used for anything other than holding drywall, then the joists need to be the right size with the right hangers...24'x24'? Without a center beam, you are looking at using 14' engineered joists at 12" OC.
Lots of A35's and LPT4's
The list goes on and on......
Please tell us that you have some detailed plans for this?