The latch is recessed into the door edge and is operated via a spindle.
It is available in various sizes enabling the handle itself to be located either closer to, or further from, the door edge.
Marking out the latch position
Mark the centre line of the lock edge of the door, and mark a horizontal line crossing this at the appropriate height. Continue this line from the door edge, back along the face of the door using a tri square.
Drilling out for the latch barrel
Measure the diameter of the latch and select a drill bit which is fractionally larger. Drill into the lock edge to the overall depth of the latch at the crossover of the two lines marked.
Make a note of the required depth and mark this on the drill bit with a piece of chalk. This will ensure that the rebate is not drilled too deeply. This is particularly important when you have a panelled door since there is often not a great deal of room to spare in the width of the stile ( the vertical piece of wood forming the edge of the door). Over drilling would result in the hole breaking through at the inner edge of the stile.
Hold the drill level and aim it along the centre line of the door to ensure that the hole runs true ( not out through the face of the door!)
Drilling for the spindle
Now, place the latch flat against the face of the door so that its plate is flush with the edge of the door. Mark the spindle hole of the latch with a pencil, crossing the horizontal line previously drawn on the door face.
Using a drill bit a little larger than the diagonal of the spindle, drill through the door at this mark. Take care as you reach the far side, as excessive pressure on the drill will cause it to split.
A neat trick here is to stop drilling as soon as the bit begins to break through the surface. Then, work from the other side.
Recess for the latch plate
Slide the barrel of the latch into the hole positioning the plate against the lock edge and draw round it. Remove the latch and, using a sharp chisel, cut along this outline to a depth equal to that of the plate. Make small cuts across the grain within the outline and pare out the waste with the chisel. Holding the barrel of the latch, offer the plate into this cut out to check that it fits neatly and is flush.
Mark the screw positions, remove the latch, and drill pilot holes.
The countersunk holes for the plate screws often have protrusions on the back of the plate. These can easily be accomodated by boring out the pilot holes a little with a hand held countersinking tool.
Clear out all the debris from the work.
The crevice tool of a vacuum cleaner does a great job of this.
Assembling and fixing the latch
Now push the latch into its housing and ensure that it lines up with the holes for the spindle before fixing in place with the screws.
Insert the spindle and mount the handles on both sides, taking care not to force the spindle out of line as you tighten the handle fixings as this will cause it to bind.
Making a small 'starter hole' with a bradawl, fix the top left screw first. Then, making sure that the handle is square and that the latch operates freely, fasten the bottom right screw. Now fix the screws on the opposite diagonal.