In addition to supplying you with great doors, we offer qualified Door DIY advice not least with this page, detailed written advice backed up with "real" joiners video of how to proceed.
Video showing how to cut an aperture in a door:
1. Make sure the door is suitable if it is a hollow flush door rather than a solid core type door an aperture can be cut but you will have to frame out the aperture, this involves installing the framework to the internal area of the door cut out.
2. Carefully pencil mark out the required aperture size and position or height in relation to the door, always try and leave at least 125mm from the top and sides of the door to the start/edge of the door aperture, make sure the pencil marks are nice and straight to where they meet at the corners.
3. There are different ways to cut an aperture but two ways to do so are......using an electric jig saw or using an electric router, I will base this piece on using a jig saw.
4. Drill a small hole at each corner of the proposed aperture, the hole must be big enough to accommodate the jig saw blade, slowly cut along the previously marked pencil line between each corner hole, try and support the section to be removed during the last few inches of the last cut, KEEP YOUR HAND AWAY FROM THE PROPOSED CUT AND DEFINITELY BEHIND THE BLADE.
5. Hopefully, you will have sourced some suitable beading for the aperture, enough for both sides, if this is an external door it is best to use beading that is rebated on the back edge so that it very slightly overlaps the face of the door.
6. Mark each bead where required and mitre the bead so that the back edge of the mitre fits snuggly into the aperture, repeat carefully for each bead so that the bead and mitre is a tight fit, pin the beading evenly on one side and fill the pin holes, install the glass as required and repeat the bead fixing process as previously.