Click on Sites (Customers) in the Accounts menu to go to the Sites (customers) page and open the customer to which you want to add an assets. Then click on Assets and Add Asset for this Customer.

After clicking the Add Asset for this Customer button, an asset form will show simply fill out the asset form with any necessary details and press Save.

If the fields when generating an asset are not all suitable you may hide some of them and also make them necessary by going to Settings > On the Settings page, from the top horizontal menu, click System Fields scroll down and look for Assets - Scheduler section.

If you want to add more fields that are specific to your assets,

go to Settings > Custom Fields, look for the Asset section, and click "Add asset field."