To edit /etc/rc.local you will need to be the root user
(then type in the root password when asked)
As for rc.local, you just type in the lines as you would on the command line, one per line. Where you could run into a problem is if what you're running asks for input (Like 'press enter to continue'). If there's a man page for the app, or documentation of some kind, see if there's a way to suppress prompts, if you can call it with a flag or something. Otherwise if you put it in rc.local it's likely that your boot will pause at that point and you'll have to provide the required response. I suppose that would still be better than what you're doing now.