RULE NUMBER ONE - blades HAVE to be OFF the mower to be properly sharpened.
It's easy to get them off and on.
There are a number of devices (blade clamps) that do this as will a short length of 2x4. Although we do not do this and will not advocate this - if you have to ALWAYS tilt the mower carb side up UNLESS instructed otherwise by your manual. If a tractor or Z-T, lift the deck.
As a professional, we NEVER tilt a walk-behind mower - we use lifts for tractors, mowers (walk-behinds) and Zero-Turns. All walk-behinds are lifted flat and never tilted. We use clamps. Remove the spark plug lead (MANDATORY!!!!!!!!!!!!!!!!!!!). Remove the blade. Clean the blade, sharpen the blade, and accurately balance it (all of which is done using professional tools - no nails, no files, no right angle grinders, NO HACK methods)
For more info see the thread in general info re. Pro BLADE SHARPENING.