More in Windows Walls and Doors
Lay down some drop cloths to protect the flooring before you begin. Then, start by carefully prying off the existing trim. Use a pair of pliers to pull the nails out from the back side of the trim in case you want to reuse it later.
To remove the door jambs, use a reciprocating saw to cut the nails holding them in place.
Remove the furring strips and then use a pry bar and hammer to begin opening up the wall. Once it's open you can see what kind of framing you have and determine if there are any hidden problems. Check for electrical outlets, switches, heating vents, etc.
Break open the wall until you can see the stud. Then cut the plaster with a reciprocating saw using the stud as a guide. Tear the big pieces off as you go along. To lengthen the opening, reveal the existing header to ascertain what kind of framing you have. You may opt to remove studs later in the project, but you're better off using the existing framework.
Make a temporary support using 2 x 6 boards. Cut them just a bit long and wedge them against the ceiling to support the load. Use screws to temporarily secure the brace.
Measure up 5-1/2" from the doorframe (the true width of the 2 x 6 boards) and make a mark for the cut. Use a level to carry the mark straight across to the other studs.
Use a reciprocating saw to cut the short studs above the door. Be sure to cut them off square, not at an angle.
To make the header, nail together a sandwich -- two pieces of 2 x 6 with a piece of plywood in between. Add contruction adhesive between the pieces for extra reinforcement.
Set the header in place as shown in the image. It should just slide into place if it's cut correctly -- you may need to do some minor adjusting with a hammer. If there is a gap between the header, you may need to insert some shims before you nail the header into place.