How to enter accounts receivable and allowance for doubtful accounts in t accounts to determine balances

There is nothing to enter into the receivables. You don't touch that account. The allowance for doubtful accounts is a contra asset account which offsets the balance of the receivables. But this amount is not taken directly out of receivables, because it isn't any particular customer's account; it is only an estimate of the dollar amount they estimate to be uncollectible. That's one thing we use contra accounts for.

The balance of the allowance account is figured the same way as any other t account. Balancing an account is balancing an account. If you have a credit and you put in another credit, it's added.

If you're having trouble figuring out what to actual put into an account, then ask that. I only see you asking about the balances.