the best way is to de-glaze the door,fit frame square and level into opening,then fit the glass,using packers to sqaure the sash up,you do htis by placing a packer on the bottom at the hinge side,then put glass in on top of it,lift the door sqare then place a packer on top of glass on lock side,use more packers on top until square,then place packers down each side of glass.If you want to take door off hinges,if it has butt hinges,open door and look at the part of hinge that is hidden when door is closed,there should be a small hex grub screw,undo them on all hinges then knock the pin that goes through the hinge up from bottom using hammer and screwdriver,once all pins are out the door will be off the frame and you can fit frame then hang the door once frame is fitted,but you might have to re-pack glass once fitted if you do it this way so first option is better,hope this helps
shaun www.sdwupvcrepair.co.uk, November 2008