Entries for Accounts receivable:
1) We debit accounts receivable when we sold items on account and the entry would be

example sale on account for $100,000)
Accounts Receivable 100,000
Sales 100,000
2) when we collect portion of this sale we credit accounts receivable to reduced it by the portion already paid and entry would be (say paid 50% of the account)
Cash 50,000
Accounts Receivable 50,000
Allowance for Doubtful accounts are provisions set for certain portion of receivable which the company estimates would be uncollectible. The entry would be (example $5,000 will be considered uncollectible)
Doubtful Account Expense 5,000
Allowance for doubtful Accounts 5,000
In the Balance sheet this would be presented as follows based on the given figure above
Accounts Receivable 50,000
Less: Allowance for Doubtful accounts 5,000
Net Receivable 45,000
Hope I answered your question.