The one thing I always recommend to ensure a smooth Lead conversion process is to deduplicate your Accounts, Contacts, and Leads ahead of time. This will help you reduce the amount of records being compared within your Convert scenario, not only making this faster, but easier to review as well!
I would highly suggest taking a moment to review our Dedupe Playbook which reviews this process from start to finish and even provides a few examples of how to best maneuver the deduping/conversion world 🙂
Additionally, you could check out our recorded training session hosted by one of our awesome CSM's. These videos are a great place to start for any module.
