You generally need your own itemized list that has sufficient detail to help you determine fair market value, such as brand, condition, date acquired, how acquired (gift, purchase, etc.) and purchase price.  You can determine fair market value based on prices for similar items from eBay, Craigslist, or local thrift stores and consignment stores.

ItsDeductible has values that are national averages and may not apply in your area.  If you want to list each item separately and use ItsDeductible values, you should make at least some effort to determine that those values are reasonable for your items in your area.  You can also determine the values on your own and enter a general custom listing ("used clothing, $75, see list for details").

Then you also need a receipt from the charity.  The charity is not required to state a value and usually won't.

The IRS is cracking down somewhat on people who make large donations and only get a blank receipt form from the charity.  How you can you prove that the items on your list are actually the items that go with that blank receipt?  For large donations you may want to get the charity to sign your itemized list.