Not overly hard but tough to find the glass to do it. One side of the frame has a rubber to "hold" the glass in, outside for the fixed glass, inside for the sliding one, pull out the rubber and you will see that the remaining glass, be it in small pieces will be stuck to the sealant, note the distance from the edge of the glass bits to the frame...1/8" for clearance. This glass is called tempered or safety tempered....for your safety! With a piece of cardboard on the frame side, trace onto it from the other side to get your visual opening or day light opening. Add to that size the amount of glass you scraped off to get the actual glass size. Also measure with a tape measure the sizes you traced onto cardboard and write them down. As a rule the glass is square or rectangular with only the corners being round. So now you have the shape and the corner curve, bring it to a glass shop with some of the glass bits so they can order either clear, shaded or mirror faced glass to match and thickness ( 3mm or 1/8" most likely), in the intirum get a piece of plexiglass from the HD or Lowes and try your hand at being a fix it guy
Len, journeyman glazier (a glass guy)