Start by going to your project's directory and type:
$ npm install --save @thetta/core
This will install Thetta core to the node_modules directory.
You can then use Thetta core from your Solidity contracts by importing it like this:
pragma solidity ^0.4.24;
import "@thetta/core/contracts/DaoClient.sol";
contract YourCompany is DaoClient {
// your business logic goes here
}