Before deciding to open your hive be aware that bees dislike the smell of newly mown grass, it can make them agitated and more liable to sting you. They are also similarly affected by the type of cool heavy air that presages a thunderstorm, so again delay opening the hive. However, once you have decided that the conditions suit,start your smoker and have a heavy cool smoke issuing from your smoker, making sure that there won't be any hot coals or chunks that will be blown out. Having your veil on would be helpful as well. Blow a few puffs into the entrance of the hive and wait a minute or so, then remove the cover. Open the cover slowly and carefully using a pry tool (my bees like to build a lot of burr comb upside down on the cover) and blow a few puffs of smoke on top of the frames and underneath the cover. The bees should retreat to the comb. Lean the cover against the side of the hive, never place it flat on the ground. This will prevent trapping or crushing the bees.
When you are ready to close the hive, puff some smoke over the tops of the frames again to keep the bees below the cover. Then position one end of the cover on top of the hive and close it like a hinge. This will help prevent the bees from being crushed under the cover.