We suggest using our WooCommerce shipping rates plugin to override your WooCommerce shipping settings with ours.
If you want to do it yourself then please refer to the WooCommerce FAQ for instructions on how to use their product shipping classes. Since Printful offers a variety of products (t-shirts, posters, canvas etc.) with each of them having different shipping rates it's best to create classes for each product type. We also recommend choosing "per item" shipping setting as our pricing is charged "per item", with an exception for posters.