I found the various instructions over at MongoDB to be baffling.
Starting from here each page links to 5 other pages that you should read, ad-infinitum, and there seems to be a lot of confusion over the 2.6 instructions and pre-2.6. It all seemed much more complicated than need be.
So here's the short version:
Install MongoDB, I'm not here for that, their instructions are good
Make yourself a config script, here's mine. You'll be able to login and do things even with auth=true because there is not currently an admin user
Fire up mongo pointing to that config like so:
mongod --config /usr/local/etc/mongod.conf &
Now we go in, and create an admin user
Change to our application database
Create our app user
Log Out of mongo and test the user
mongo -u appUser -p appPasswrd appDatabase
That's it, if I'm wrong, or there is another best practice, let me know.