You can cite resources for a Remix during the second step of the Record upload process. 

You will see a "Suggested Resource" filter where you can choose from Suggested Records to cite, or you can search for specific Records using the search bar that lives above the Suggested Resource list.

Additionally, you can select Resources that you have saved in your Private Collections by clicking on the Private Collection tab in the Resource widget. To edit or add additional resources to published Records, you can access the Resource widget by clicking on the Edit button on a Record and selecting the Edit Resources button located at the bottom of the Record Edit form.