Good question. There is a great webinar that you might want to check out that goes into more depth on data collection; “How to Successfully Collect Process Data for Your Lean Six Sigma Project” (Link: https://goleansixsigma.com/webinar-successfully-collect-process-data-lean-six-sigma-project/).
Your baseline is basically the most recent data reflecting the state of your main metric. So if you were going to reduce the amount of rework then you want to measure how much rework is in the process now. If you are collecting it by hand that might mean asking people to keep track of each time they have to rework a unit. Then use the total number of units for that time period to get a baseline of the % of Rework.
You can also access the free Yellow Belt Training (https://goleansixsigma.com/free-lean-six-sigma-training/) to review the Measure Phase in more detail.