There are quite a few different methods of going about this, and each have their positives and negatives.
First, you have your "above ground" bunkers. These are normally sheets of plywood or similar material, fashioned together into a horizontal "L" or "V" shape as to cover more than one flank. These are generally cheap and very simple to make, and are also portable.
Then you have your regular or "sub-ground" bunker. This bunker is normally started by digging out a 4 foot x 4 foot area, 2-3 feet deep, and then mounding or sandbagging all the dirt on the side. You should also dig a small entrance trench in the rear. If you decide to stop right now, you have a "foxhole".
After that, you simply add your plywood or similar material to the sides, and if you choose so, add a roof and then cut "gun ports" into the sides of the wood. Remember to leave the back entrance open. The negatives of this type is that is a lot of work to produce, permanent, and requires maintenance. However, you can feel satisfied that you have a real bunker, and will most certainly add new elements to your gameplay.
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Use metal, cardboard and top it with spray paint camo (for cool looks) and leaves for an authentic war feel only do that if you use spring airsost guns under 250fps. I use high aeg with 380fps and my guns tear rite through 2 layers of cardborard. If you own high grade AEGs like me dig a 3 foot trentch and put logs in front of the hole. or if you have chopped wood (15 pieces) dig a 2 foot hole then put two strong sticks in the ground one in front of the other and burry the sticks. then get you chopped wood logs and stack the on top of each other. The sticks should hold and if you want to make it really tall use 4 sticks
Use large plywood sheets, put them sideways on the ground so that they form a rectangle, make sure you have an exit/entry at the back.
or hope it snows