An inexpensive hand-tool and not-a-lot-of-skills can have you cutting glass in no time!
Step 1: Parts Layout
(1) glass cutter (start at ~4$ and go up from there. The type used here is of the ~4$ variety)
(2) sturdy straight edge (the wood plank in this case)
(3) some glass or mirror to cut
Step 2: Score the Glass
first, wet the cutting wheel on the glass cutter (with water, oil, or saliva)
Next, set the straight edge along the cut-line. Place the cutting wheel at one end of the cut-line, apply downward pressure and pull the cutter along the cut line. The cut will work much better if the glass is scored in a single pass, so take care to go the ful length of the line with constant downard pressure (and practice practice practice)
Step 3: Tap the Glass About the Score; Finish Cut
Use the ball end of the glass cutter and tap (whack) around the scored glass. The Glass should break right along the line Turn over glass piece and tap on the backside, too.
Sometimes with stubborn glass grab the glass and bend along the scored glass. Don't force it. trying to "fold" glass is a recipe for disaster..but it helps sometimes.