How do I put a window in my front door?

Are you handy with carpenter skills? Remove your door, set up on saw horses. You'll have to cut out the opening, route out for the glass, get tempered glass made to correct size, less 1/8" in both directions, install glass.

You can cut a hole in the door where you want the window. I would build a picture frame style insert for the door. You can insert the glass into the frame and put a wooden frame insert to hold the glass in. I would check with a local glass company and maybe buy a double pane pre-made unit. That will help with heat and A/C costs.

It may just be easier to search Ebay or Craigslist for a used one. By the time you buy the glass, trim, paint, sandpaper, etc. and counting your labor and time (projects are always harder and take longer than you think they will) it may be more logical to replace it. I know you said you couldn't afford a NEW door, but the Home Improvement stores also have Clearance sections worth checking out. Also, check your local thrift shops or Habitat for Humanity Store. Good Luck!