Read it. A little every day until you've passed your eyes over all of it. Make notes about mysterious things. Check them off once you've figured them out. Try

Read it. A little every day until you've passed your eyes over all of it. Make notes about mysterious things. Check them off once you've figured them out. Try to find the 'business case' database. You will fail, nobody has one. Make one then, while you're reading the code.