Before enabling the shipping method you should configure the shipping settings from the Stores menu > Settings section > click on Configuration > expand Sales > click on Shipping settings.
Expand the Origin section. Enter the address from which the shipping will be initiated. Usually this is the address of the warehouse where the products are stored. Afterwards open up the Multishipping settings in the Sales menu and expand the Options section. If enabled, the option Allow Shipping to Multiple Addresses allows goods from a single order to be shipped to different addresses. It is useful when a customer orders many products of a single kind, like post cards or pens for example and wants them to be delivered to multiple addresses. You can define the maximum number of allowed shipping addresses in the Maximum Qty Allowed for Shipping to Multiple Addresses field.
For example, to enable the USPS shipping module, you can go to Stores menu > Settings section > click on Configuration > expand Sales > click on Shipping methods and navigate to the USPS section.
Then set Enabled for Checkout to Yes. The Gateway URL is needed to access USPS shipping rates. The field is preset by default, and normally does not need to be changed. The next step is to enter a Title for this shipping method that will appear during checkout. Afterwards Enter the User ID and Password for your USPS account. You may also adjust the Container to the type of packaging usually used to ship products ordered for your store, Size of the typical package. etc. In the Free method field you may choose the method you want to use for free shipping. You may also select None.
At the end you can define the countries for which the shipping is supported and the default error message in case the service is refused. In the Sort order field you should enter the position of this shipping method compared to the other shipping methods offered to the customers.