Shut off the gas supply to your light. Depending on your style of lamp, you will have a gas valve at the bottom of the lamp post or a switch near the top of the light. Consult your specific lamp manufacturer's documentation to find this switch.
Remove one of the glass panels of the light. These glass panels will either slide straight up and out of the way, or open like a door.
Examine the mantle for damage. The mantle will look like a small, mesh bag that covers the burner inside the light. If the mantle has any holes it should be replaced. Depending on the style of mantle, they are secured to the burner lip by either a ceramic fitting that slides over or by a small drawstring that ties around the lip.
Install the new mantle if necessary. Be careful when installing the new mantle, as they are delicate.
Turn the gas supply that feeds the light back on. Strike the long kitchen match, and hold the flame near the mantle. It is not necessary to touch the mantle, as it will ignite without contact.
Replace the glass panel or door as necessary.