Don't rest the bike on the grass, as the ground is soft and will absorb some of the energy that should properly be absorbed by the cotter instead. Put the block (I use a wood block with a short piece of gas pipe embedded in it to receive the cotter) on a solid surface . Concrete is good. Arrange the frame so the crank rests on the wood block and is properly positioned. Then rest a crow bar (or other suitable piece of steel ) sideways on the end of the cotter. The crowbar will help preserve the cotter. Strike the crowbar forcefully (don't be bashfull) and it should pop out. I don't think I've ever used more than two blows.

Good luck!