Sally, if you can change one part of your "work list", you can do it. You say "have it cut". Because there will be variations between windows, etc., you will pretty much have to do the final cutting to length. Of course, I have not seen your house and the kind of siding that you have will also be a factor. I'm sure that you will not want to get involved with cutting back the siding to install the trim. If that would be necessary, you would want to have the work done.
One part of good news is that you will not have to sand the molding. It comes normally with a good enough finish that you do not have to sand before priming and painting. But you will want to prime the back of the molding before you install it for a little more protection from the weather.
I don't know where you are located either, but you will probably want to wait until spring or summer to do the work. (Don't start on your first window, before you have any experience, if the forecast is for heavy rain the next day, just in case you run into unexpected problems.) But now is a good time to do your homework. There are many TV shows on HGTV, TLC and PBS that show how to do this, but they also have to show many other things. Then, again, you may get some ideas for other improvements that you might want to consider.