How to handle credit note on unclaimed ITC?

Hello,
I run a small business and file my own GST. I need some help with a few things.

1. I claim ITC for most of the invoices and sometime I ignore a few invoices because they are non-business purchases. Is this the right way to go about this or should I mention the non-claimed ITC amount somewhere in the 3B?
2. Last month I ignored claiming ITC for an invoice because the product was returned in the same month but the invoice still populated on my 2B. This month there was a credit note for that amount and reduced my ITC for this month. How I do avoid this?

Thank you in advance.