First, download your transactions from your Mint account. You can find the export link on the transactions page, in very small type at the bottom right, or by searching the page for "Export all".
Then run the script, like so:
mint-to-ledger [-h] csv_file ledger_file
There are a couple of problems with the resulting ledger due to incomplete information on Mint's part. In other words, they may or may not ever be fixed.
Mint represents a transfer between two of your own accounts by assigning the category "Transfer" to the transaction appearing on each account statement. The script interprets this as one transaction from the source account to "Expenses:Transfer" and another from "Income:Transfer" to the destination account. It should really be expressed as a single transaction from one Assets account to the other.
I've special-cased this for cash withdrawals since Mint provides the category "Transfer for Cash Spending", but haven't dealt with the transfers to and from accounts with arbitrary names. It seems possible to allow for user-defined categories which are interpreted as transfers rather than expenses, but I haven't tried to implement this yet.
If an account ever changes in value due to something other than a transaction (for example, market movements affect the balance of your brokerage account), Mint fetches the correct balance but doesn't indicate it in the transactions file. I doubt I can ever fix this since the relevant information just isn't provided.