Having just looked at the pictures on my PC I can now see that you have already drilled the clearance holes both for the spindle and for thumb turn. Obviously you need to fit the latch or lock into the edge of the door.
Once that is perfectly positioned, the spindle should be inserted through the latch/lock. With the spindle in place, the first back plate can be positioned so that it is parallel with the edge of the door. Do this using a small sliding set square.
Care must be taken to make sure the spindle is straight and level.
I would drill 2 small holes into the door through the fixing point in the back plate. Just to mark the centre point for the fixing screws. Then get two small screws .. smaller than the actual fixing bolts supplied and screw the back plate to the door, not winding the screws all the way home, just a partial fixing.
You can now offer the rear back plate up to the door and check to see if it sits nicely in place. Check that the latch operates without binding. If it does, you need to adjust the 2 small screws. If all is well than do the same with the rear plate.
When that is done remove the small screws and get your clearance drill and drill most of the way through the door.. straight and level, before drilling through from the other side and making the two holes meet.
Insert your fixing bolts and tighten .. that it !
For some one who has to ask for help, I am impressed that you have drilled the spindle holes larger than the spindles.
Looks like a professional job to me ! ;-)
Mines a Sauvignon Blanc..