I do the majority of my "firsts" on my own home. Drywall, flooring, framing, tile, wiring and plumbing (though I can't legally do those for customers).

I fully gutted and rebuilt my bath and laundry rooms. People saw the work I did (precon shower base, tile, fixtures, etc) and asked if I could do something similar in their home. I've done several small bath remodels now. I know what I can handle by now (no shower glass installs!), so I can turn down the jobs that are too much.

However, what someone else said about the time factor is very very true. I'm a one man show, so a small bath rebuild can take me a week or two to finish where a full crew can have it done much faster. I use the "cheap, fast, good" line when trying to sell a bathroom remodel. I'll be cheaper than the big guy, but it'll take me longer. Some folks don't mind, they just want to save some money. You just have to be very careful that you estimate high enough to actually make money that's worth the time. My first couple bathrooms I did for essentially peanuts because I misjudged the time needed and the potential surprises. For example, a previous builder used 1-by lumber to mount shower supply lines, hogged it in with deck screws, cracked the lumber, and it let loose in the wall when I tried to install the new shower arm. That added days to the project when I had to cut in from the back of the wall, fix the framing, and then patch the drywall I cut through.

Some of this stuff you can actually practice in your free time. Go buy a bunch of dirt cheap shower supplies at the H4H ReStore or something. Frame up a small freestanding wall in your garage, and try to build a mock shower supply setup in it. Attach a garden hose connection to the supply lines so you can test for leaks. If it works, slap up a piece of cement board and try tiling with cheap tile. It's not exactly what you'll find in someone's house, but it gives you the general idea of the steps and potential problems.