Approach using PrixFixe the same way you would Alacarte... the gui will walk you through everything you need to know, and if it needs a file path (for the program you want to run, and the icon you want to use) it will popup a file navigation window for you to pick the program/icon.
The following are each question PrixFixe will ask:
1) Program name
This will end up being what the program is called in the favorites menu. It is also important you choose something UNIQUE here. i.e. -> If you have "firefox" as an application you are making the menu for, don't name it "firefox" in fear you may overwrite the official menu item for it in "applications". Something like "firefox-launch" or "firefox-fav" would be better2) Program Description
Enter whatever you like here, to describe the program you are making a menu entry for3) Program Icon
Choose any icon you would like to use for the menu item. If you are making one for a program that already exists, you can choose the same one if you want. You can figure out what other programs are using their icon as by looking at '/usr/share/applications/[program_name].desktop'4) Program Path
This will be what the menu item executes when you click it. You can figure out what other programs call by looking at the same file I listed above, or by trying this command: 'which [program_name]'5) Program Type
These are the different sections that exist under "Applications", choose whatever category you think it belongs in6) Arguments for the program
Now that you chose what the program path is, do you want any arguments for it? i.e. -> (for 'ls', something like '-al' may be wanted here)7) Terminal usage
If this program does not have a GUI that will run (a command versus a GUI application), you will want to choose this option so you can see the output in the terminal window
Give it a try, PrixFixe should be pretty hard to "screw up", unless you choose an application name that exists already
Let me know if you have any other questions.