The steps required to create an instance of an externprototype are:

Read in the externprototype. You can read it in from a file or a buffer using the readAllVRML method of SoDB.

Find the externprototype definition from the externprototype's name using the static method findEXTERNPROTO in SoBase.

Create an instance of the prototype.

Set the values of the fields.

Finish the setup of the instance.

Reference