A mark-up exercise will literally be you needing to scrutinise every little detail of a document. You'll need to look out for anything out of the following:
- Typos
- Mispelling
- Inconsistencies in spelling (e.g. names like Thompson/Thomson)
- Grammatical errors
- Transposed numbers
- Incorrect dates
- Incorrect headings/titles/subject lines
- Incorrect identification of parties/people (e.g. referencing the client as the other party in a deal or vice versa)
- Language - where sentences or bullet points are ambiguous, unclear, or over-written
- Structure - where things could be reordered to make a more logical/clearer layout (might be particularly relevant in this instance if there is a timeline aspect)
I haven't seen a "timeline" exercise before, but I would expect this to be based on what needs to happen first and then deciding the order of actions to ensure success. This will just need some common sense to work out what will have to happen before a particular outcome, or where matters/parts of the puzzle are dependent on others.