We will assume you have gone through steps #1 to #3 in adjusting a rear dérailleur. This is the last and easiest step. You will require a bike stand or a person willing to hold the rear wheel in the air.
Due to spring tension the rear dérailleur prefers to stay in the smallest cog ( assuming there is no cable tension ). Cable tension lifts the dérailleur into the larger cogs. Hence if the dérailleur does not move into larger cogs it needs more cable tension, achieved by turning the barrel adjuster counter clockwise. If the dérailleur does not move into smaller cogs it has too much cable tension, and the tension must be relieved by turning the barrel adjuster clockwise.
In short our procedure ( or algorithm ) for adjusting the shifter is as follows. If the shifter is in a smaller number gear than the chain ( for ex. chain is in #2 cog while shifter in #1 ) one should turn the barrel adjuster clockwise. On the other hand. If the shifter is in a larger number gear than the chain ( for ex. chain is in #2 cog while shifter in #3 ) one should turn the barrel adjuster counter clockwise.
Step #1 View the barrel adjuster on the rear dérailleur.
Step #2 Put the bike in a stand or have a partner hold up the rear wheel.
Step #3 Put the shifter in 1st gear and rotate the pedals. If the chain is not in the biggest cog we will assume it is in a smaller cog due to the limiting ( H & L ) screws do not allow it anywhere else. Turn the barrel adjuster counter clockwise & rotate the pedals until the chain is in the largest cog.