How do I mark touchers?

Under law 42.2.10 you must chalk all touchers and remove chalk from non-touchers as soon as they come to rest. Bowls SA's Markers Guide adds:

• Tell the players before the game that you will mark touchers as soon as they stop.
• Mark the toucher neatly on the running surface, not across the rings, dimples or emblem, as soon as it stops and before the next bowl is delivered, even if the next player is already on the mat (they are not yet in possession of the rink).
• A bowl that stops and then falls onto the jack before the next bowl is delivered is also a toucher: mark it as soon as the next bowl is delivered.
• Erase chalk from any played bowl that doesn't become a toucher. Don't lift a bowl to do so; if the mark is underneath, mark an "O" on it.
• If marking or erasing would disturb the bowl or the head, indicate to the players that it is a toucher (or non-toucher) instead.
• For a toucher or the jack in the ditch, mark its position (law 42.2.13): an indicator against or on the bank in line with it (white for the jack), and lines drawn in the sand or with chalk around it.

If a toucher is neither marked nor nominated before the next bowl comes to rest, it stops being a toucher (law 15.3).