To use quick heal, you must have at least one Med Pack in your inventory. Your life bar, visible on the back of Isaac's Rig, must also be at least partially depleted. Press B and Isaac will use a medpack to quickly heal himself. This should unlock through normal game progression. You will be using quick heal a lot. If you wish to unlock the achievement as quickly as possible, you could simply fill your inventory with medpacks (through looting or crafting), allow enemies to damage you, and keep spamming quick heal as required until the achievement unlocks.