A first engagement is easier to assess when the deliverable is concrete and its boundaries are visible. Choose a real problem within your present ability to address.
Agree what the client will receive, which inputs they must provide and how completion will be reviewed. Keep important exclusions clear enough to prevent an assumption becoming an unintended promise.
Demonstrate the work
For a fictional reporting engagement, a useful scope might cover configuring one existing workbook for one workflow, testing an agreed example and handing it over with instructions. A claim to transform the entire operation would require a very different basis.
Use an honest sample to establish understanding. Never present fictional results as evidence of earlier clients. If the work requires expertise you do not have, arrange suitable help or decline that part.
After delivery, ask what was useful and where the process caused unnecessary effort. Review your own capacity and the actual work involved before repeating the offer. The first engagement should create a dependable piece of work and a clearer understanding of what you can responsibly promise next.