yeah Josh is right, there are five million answers to that question - but more than anything I try to hammer a couple of points into people before going into this:
1. Don't sync your Marketo production to a Salesforce sandbox to just "try it out". You'll have to blow your Marketo instance and start over with everything. Marketo and Salesforce mate for life, you can never undo the sync and swap it for another Salesforce instance.
2. On this page of instructions there's a bullet point here about ensuring that new lead and contact fields you create are mapped: Step 1 of 3: Add Marketo Fields to Salesforce (Enterprise/Unlimited) - Marketo Docs - Product Docs
Often people ignore this step. This is referring in this doc to new fields created for the Marketo sync, but it applies to ALL Salesforce fields. Ensure all the fields you want to convert from leads to contacts on lead conversion are mapped before doing the initial Marketo sync, otherwise you'll end up with two sets of fields and spend forever merging or making up for them. It's the #1 most painful error I run into time after time.
Otherwise, just follow the instructions carefully and do it step by step. Don't skip a step just because you don't understand how to do it and assume it'll be OK (I see that a lot too).
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