How do I write an SQL script to copy the securables, memberships and owned schemas of a current database user?
When you restore a database all the users and privilege assigned to users (including database role membership) from that database get restored at the destination server.  If you already have a login in the destination server with matching Server-Level Identification Number (SID) then you do not have to do anything.  

If you do not have a login you will need to create one:


Copy login from source to destination using this or this.


You have have a login with different SID you can match those using solution from here.