First, if you have Red Alert 3 opened, close it.
Open C&C Online client
Make sure Red Alert 3 is not hooked yet, by going into the Hook drop-down and making sure Red Alert 3 does not have a checkmark in front of it. If it does, click it again so that it becomes unhooked.
After that, open Red Alert 3 normally, and wait for it to load to the Main Menu.
Then switch to the C&C Client and then click the Hook drop-down and select Red Alert 3 to hook it, it will take time, and may freeze for a while. When it unfreezes it will display the Red Alert 3 with a checkmark in front of it.
Now close Red Alert 3 and close C&C Client too.
If all goes well, when opening Red Alert 3 through Steam, it should now open C&C Client instead.
Now click the Red Alert 3 button on C&C Client to launch Red Alert 3.
If the game opens with no errors, and loads it to the Main Menu just as before, the game was successfully hooked.
Done! You can close everything again if you prefer.