For purging the line, literally just open the valve slowly and let it bleed to the atmosphere. Even short lines take longer than you would think. On my new house, i had to purge the dryer and furnace lines for several minutes. the propane has an oderant, so close the valve when you get a strong smell. Even then, the line won't be completely purged. The appliance should ignite but the flames will be slightly orange in color due to the air/propane mix in the line. Let the appliance run and it will purge out the rest.
obvisously, when purging, don't have open flames, hot spots, be well ventilated, wait, etc.. no warranty expressed or implied.