Step 1: Parts Layout
(2) sturdy straight edge (the wood plank in this case)
(3) some glass or mirror to cut
Step 2: Score the Glass
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
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.