Cheat Mode
Start the game as usual, open up the console with [~] key and enter the following command (it is case sensitive):

sam_iEnableCheats=1

Once enabled, close the console (press [~] again) and bring up the main menu by pressing the [Esc] key - now you can enable the basic cheat menu by pressing [F1]. Repeat as often as needed just remember you have to be in the main menu to trigger the cheatmenu with [F1].

If you want to use the developer cheats simply use the following command instead:

sam_iEnableCheats=2

You can also disable the cheats with:

sam_iEnableCheats=0

Cheat Mode (alternative)
Hold [Left Shift], then press [F3] [F4] [F3] [F3] [F4] at the main menu. Then, press [F1] to display the cheat menu.