[Imp]

Do the following
1) Set the side you want fortifications for to defend
2) Buy bunkers under the Misc tab for units.
3) For the rest spend points on by pressing the button below the tabs for Inf, armour, arty, misc etc on the buy screen.
4) Deploy & you will see a button buy the entrench all (spade) for spending your fortification points. Use this to place them.

Note on bunkers when reload in editor facing is often lost so last thing you should do once completed is change the facing for them if its not default.