On my most recent day off I decided to tackle the bathroom water supply.
For the most part, I decided it would be easiest to basically start from scratch, rather than work with the existing crap that was in place. This was especially true of the hot water supply which was 1/2" copper and needed to supply: the bathroom sink, the shower, the laundry room sink, the clothes washer. A quick note about sizing: an easy rule of thumb is that a 1/2" copper line can supply 2 fixtures, but there are plenty of tables available for you if you want to do the exact calculation for your situation. For cold water there's no downside (other than cost) to erring on the high side. For hot water, the downside of putting in a pipe too large for the fixtures you are feeding is potentially having to wait longer for the hot water to arrive at the fixture.
After cutting out the section of hot water pipe that looked like a mess I cut the new supply lines to size. Working around existing wiring, piping, HVAC lines, etc. can be the biggest challenge when plumbing and plumber's are somewhat notorious for their liberal use of the reciprocating saw.
(no visible cuts into joists here, but you see the maze of existing piping to work around)
This is precisely why planning out your layout beforehand is a good idea. Just use common sense - straight lines and 45 degree bends are better than 90 degree turns because they don't slow the water down or increase likelihood of water hammer, try to avoid the aforementioned obstacles, running pipes under joists is easier (and better for the structure) than running them through them, and try to keep in mind where you will strap the pipe. If you must go through joists or studs make sure you're aware of the notching/hole guidelines. Studs: on bearing walls studs can be notched 25% of their width and holes can be up to 40%. On nonbearing walls the notches can be 40% of the stud width and holes can be up to 60%. Joists: No notching in the middle third, notching at the end can't be more than 1/4 of the joist depth, notching elsewhere can be up to 1/6 deep with a length no more than 1/3 the depth of the joist. Holes have to be more than 2" away from the top, bottom, and other holes and can only be 1/3 the size of the joist depth.
A word about securing pipes. There are a lot of different kinds of straps out there - plastic nail on "talons," plastic screw on straps, copper straps, bell hangers and probably more.
I like to keep each of them in stock in case one suits the application better than another. Bell hangers are harder to find in some big box stores like Home Depot, but they're sturdy and keep the pipe off the lumber which can be nice if wire or something else is already running on the same path. Generally speaking, you want to secure the pipe every 6' or less. Don't use nails (ever) or those nail-on wire hangers in seismic zones.
After I was done working on the hot water line I started work on the cold water line. I wanted to do them separately so I wouldn't get confused.
Working on this can be somewhat zen like. Plan a creative and smooth path from A to B, cut pipe, clean all mating surfaces, flux, heat, apply solder, and clean joint. It takes a long time, but it's my favorite part about plumbing.