The best way to install an opening in a load-bearing wall is to first install some type of support system to hold up the trusses or joists in the ceiling near the wall. Because the wall is load-bearing, if it is removed before supporting the surrounding ceiling, the trusses or joists could give in, causing the ceiling to collapse.
Trusses and joists are essentially the same thing. Trusses, however, are a series of boards and beams arranged in such a way to support a roof, whereas joists are beams arranged parallel to one another to support a second story floor. Either way, these need to be supported when removing a portion of the load-bearing wall. One way to do this is by cutting sections of 2 X 4 boards that are the same length as the height of the ceiling. Arrange them to provide the support needed. Another way is to use an adjustable steel column, also known as a lally column. This tool can be adjusted to fit the height requirement for the room to support the trusses or joists above.
Once the ceiling is supported, begin knocking down the portion of the load-bearing wall that needs to be removed. It is recommended that this be done with a sledgehammer. Once the desired area of the wall is clear, install a laminate beam to support the excess wall. A laminate beam can be constructed by placing a 1/2-inch piece of plywood between two pieces of 2 X 8 boards. All three pieces need to be glued using wood glue, and then nailed together. This beam can then be used to support the top of the new opening in the load-bearing wall. Use other pieces of lumber to frame the sides of the new opening and support the laminate beam.