Ground db and Mantra

Hi, I’m trying to Ground a Collection but I can’t understand how it works.

My app is under the mantra architecture.

Here is a sample code working online to retrieve a list of elements :

if (Meteor.subscribe('_boulders.list', selector, sort, perPage).ready()) {
        let boulders = Collections.Boulders.find(selector, {sort: sort, limit: perPage}).fetch();
            onData(null, {

How can I add ground db capabilities so that I still have my boulders offline ?