How the hell do you people do it!?

[Flying Mullet]

Here's a screen shot to help.
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You will see on the left side towards the bottom of the screen that you can put up to five units on the field, "Capacity:5 " and that you don't have any units placed yet: "Remaning: 5". Everytime you put a unit on the field you will see the "Remaining: X" count drop by one. Use the "L1" and "R1" buttons, as signified onthe top left and right corners of the screen, to move through the available troops. When you come across a troop that you want to place, Press "X" to place the troop on the grid. Make sure that your cursor is over a vacant space, or you will replace the existing troop.

Hope this helps.