I already know this transaction goes in the general journal (M49). How do I reopen an account that has been previously written off? Do I do a debit to accounts receivable/ the customers name and a credit to allowance for uncollectible accounts?

Don't know if it's too late for this... but you're correct. You're essentially un-doing the entry you did when it was written off, which was a dr to allowance and a cr to A/R. So you're entry is backwards of that and will un-do the original.