What features you decide to incorporate in your bathroom will dictate the amount of framing you do.
Expanding a bathroom's dimensions may require tearing down existing walls then erecting new ones in an adjacent room.
That work requires knowing about load-bearing walls and framing basics. So for that and more information not discussed here see Framing.
Adding a short partition wall creates privacy and defines bathroom space; like a wall between the toilet and sink. Heights vary, but normally range from 30" to 40".
Lay out the partition so it butts up to an existing wall stud or cross brace and if possible, across floor joists for stronger fastening.
It's not critical for a small partition to run across joists, but it should at the least run along one joist that serves as a nailer.
You can lay out the partition framing on the floor, fasten it all together, and then secure it to the wall, but we'll explain how to "stick-build" the wall from scratch.
Set a framing square on the main wall, mark the partition's bottom plate location on the floor. Use construction adhesive (optional) and nails/screws to fasten the plate to the subfloor and joist(s).
Cut an end stud to height, set it on the plate, and butt it next to the main wall's stud and fasten them together.
Then fasten studs on the plate every 16" and on the plate's other end. Cap the partition studs with a top plate.
That completes the wall framing, but if you're going to add a toilet tissue holder or a grab bar, add horizontal cross braces where those items will fasten.
Bathrooms can utilize almost any type of window. Roof (skylight), bay or bow, casement, awing, double-hung windows can all look nice in a bathroom.
Existing windows can usually be worked into a remodeling plan, but on occasion do cause problems.
Sometimes, an existing bathroom window is too big and doesn't afford much privacy. Installing a smaller window -- or eliminating it -- is one solution.
Downsizing a window takes more work because it involves re-framing and matching/adding siding to the exterior.
For even more how-to information see Windows: Installing & Replacing