How to fit glass panels in internal doors

If the glass is not double paned, it is usually fairly straight forward.

After you have cleaned the door and removed any of the old remnants, place the glass panes into the section. You can buy little metal triangle shaped tacks, called glazier's points, which are designed just for this purpose, and will hold the glass in place.

Once you have tacked it in, caulk/putty around the edges and over the tacks, taking care not to get any putty/caulk on the window itself or at least cleaning it before it drys.

Using a putty knife, smooth the caulk so that it forms a flat edge.

If it is not single paned, depending on the size and number of doors, it might be cheaper and easier to simply replace the doors. I broke a large exterior door once and replacing the glass would have been hundreds of dollars, but I was able to goto a shop that sells used building supplies and buy a new door for less than $50.