Ok so first thing you need to do is locate your C&C Renegade shortcut.
Right click it, and click on the tab "Find Target".
The game folder should come up.
If you are using the TFD version... In that folder you will see a list of folders of the C&C games.
Find the Renegade folder, then open it, and open the Data folder.
In the Data folder, there is a file called "skirmish.ini"
If you can't see the files extensions, right click on a empty section of the folder, and select "View/Display list"
Now you will be able to see the file types.
Open the "Skirmish.ini", which is a notepad document, and now you can edit the amount of money you start with (higher amount is more), the time (zero for infinite), weapons spawns(Yes for weapons spawning randomly in maps), among many other cool things, to play skirmish at your heart's content.
You can also add the other maps to the list.
Simply follow the example of the first map, but just change the names of the maps respectively.
Bare in mind that the first map will always play first, regardless if you change it or not.
It comes as default and will always play first, but the following maps will play respectively.