A mark scheme can be written as instructions to a marker rather than as a model answer, which suits questions where there is no single right response.
Writing one
Open the lesson from My Library and click into the question you want to work on.
Type your instructions into the Mark Scheme field.
What works
Say what earns each mark, and how many. "1 mark for naming the reactants, 1 mark for the products."
Say what does not earn a mark. "A generic statement with no reference to the passage gets 0."
Give the acceptable range for a calculation, not just the exact figure.
Keep each criterion to one idea. The marker splits your scheme into separate criteria and decides each on its own, so one idea per line marks more consistently.
See How long answers are marked for what the marker does with it.