The most preferred way to supply artwork is in a multipage PDF file.
As an example, if you were printing a 12-page booklet the artwork would be as follows
Page 1 - Front Cover Page 2 - Inside Front Cover
And so on...
Page 11 - Inside Back Cover Page 12 - Back Cover
If you need any help with this or have any questions, please email us at help@welove2print.com or use the live chat function, and we will help prepare your artwork free of charge