Step 1: Resources
you must also concider what weapons and the number of people you will be housing, because theres no point housing a sniper nest if you only have a springer pistol.
Step 2: Getting started
In woodland these should be up hill, with high barracades so that you wont have to angle you body out of the line of fire. the barricades should be long complexes so you can reposition easily. it is also good to plan an escape route and maybe somewhere to house a weapon cache along route. if you have claymores or mines at your disposal use these on the build up to the base as once inside the enemy may be looking for them and wont be in the rush between cover.
in an industrial enviorment try and make the base upstairs so that it is easier to defend, you should also try and make the doors open outwards so that it is harder to breach and and windows are small and thin walled allowing a good range of fire positions but a small target. any entrances should be able to be covered and there should be minimal chances for flanking.