When removing pedals, take great care that you don't slip of the spindle and gash your knuckles on the chain/ring.
I prefer to use a bit of gentle foot persuasion, noting that the non-drive-side has a reverse thread.
1. Put bike in highest gear.
2. Place pedal pointing to upper/rear (10:00 or 2:00 ish)
3. Place spanner on spindle, engaging fully, pointing back and up a little. Rotate pedals to ensure spanner direction.
4. Apply brakes.
5. Press foot of spanner and gradually increase pressure and weight downwards on spanner.
When re-applying pedals, smear threads with grease or copper antisieze. Note reverse thread side.
Engage threads by reversing until they click, to avoid crossing threads. Spin on by hand. Tighten with spanner.
If you have allen key spindle and not spanner flats, ignore 1-5.