Good morning Rickey,.... This is a pretty "heavy duty" project and is normally not done by a homeowner or DIY person. However, it can be done with careful planning.
Of course the work would begin at the top of the chimney. Have you made plans to have the roof opening framed in, sheated, and re roofed, or are you also doing this yourself.? Are you somewhat experienced as a handyman and do you have a variety of tools available? Do you know the pitch of your roof? What is the approximate distance from the chimney straight down to the edge of your roof? Also the distance sideways to the end of the house? On the inside, what is the length and width of the fireplace at the floor?
I know this seems like a lot of questions but if you hired a professional person to do this these are the things that would be going through his head to decide on the best approach to remove the bricks.
Get back to me with these answers and I'll get you going..
Start at the top and work your way through the attic. Once you have passed through the roof sheathing you can apply plywood to fill in the hole. Just follow the existing sheathing when nailing your "patch". You can ( with a straight steel bar) keep chipping down at the bricks (or flue and bricks) and let them fall inside the chimney as you go along. Just empty the bottom (fireplace) of the chimney of bricks as it fills up. Make sure you have a dust guard like a poly tarp to catch any dust and masonry bits at the fireplace opening.
As you empty the fire pit of bricks you will be able to reach down and break up more bricks and other masonry product as you go along eventually ridding the entire area of masonry. Just patch the ceiling after you are done and you now have a bigger, more open, floor plan. Good luck.
Hope I Helped :)