Walking around in loose sandals is impossible, especially if they don't have a back. Birkenstock straps are made of soft leather, and the nature of leather is to loosen over time. You will most likely need to adjust the straps at least once during the life of your sandals. Adjusting is as easy as unbuckling and buckling again, but if you have slender feet, you may need to make your own buckle adjustments.
Unbuckle each strap by taking the pin out of the adjustment hole in the leather strap. Do this to all straps on your Birkenstock, whether it has one, two or three straps.
Place your foot on the bed of the shoe and stand up. While standing, bend forward (if this step is physically challenging for you, kneel on the ground with one leg up as if you were proposing marriage) and pull the strap through the buckle until it feels comfortable, and insert the buckle pin in the hole. Do this with all straps.
Stand up and walk around to make sure it is snug but not tight. If it feels uncomfortable, readjust by loosening the buckle by one notch.
Unbuckle each strap by taking the pin out of the adjustment hole. Place your foot on the bed of the shoe and pull the strap until it is comfortably snug. If you have pulled the strap past the last adjustment hole, you will need to make a new hole. Mark the spot where the new hole should be (based on where you pulled it) with a pencil.
Place the unbuckled strap with the top side facing upward on a sturdy surface such as the ground or a cutting board covered with a towel. Place a nail (any size you have) on top of the pencil marking and punch through the leather by striking with a hammer. When the hole has gone all the way through, stop hammering.
Test the hole by buckling the shoe. If the buckle pin cannot fit through the hole, you will need to make it bigger by placing the nail back in the hole and twisting it around and pushing up along the thicker end of the nail. Test the hole again and adjust once more if necessary.