First off, are there blocks over the window? If so, you will need a longer lentil to support them after you cut your opening.
Typically, there will not be any blocks above the opening, in which case you can simply cut the wall opening without any additional reinforcement needed.
You will need your pit to have a drain or sump and sump pump to evacuate the rains that come in the pit. Your footing should be at least 8" thick and 16" wide with 2 #4 reinforcing bars continous.
I would recommend a concrete floor in the bottom of the pit and landscaping blocks that interlock to hold the dirt back on both sides. They can be dry stacked and are easy to reposition later, in case of erosion or movement of the dirt.