Values from Salesforce to Jira custom field

Hi, guys!
I am tying to connect external data source (SalesForce) to my custom field in Jira SD. I know there are plugins like “External Data for Jira Fields” or “Jira & Salesforce connector”! Those plugins work well but we have to pay for them. We decided to create our own connector to take a data from SalesForce and add it as a list to choose in Jira SD custom field. I understand that this is not an easy question but can you please give a hint where to move to reach this goal.
I play with SalesForce API and have a success to take necessary data. But I am stuck to insert this data to Jira SD. Can you please help with this?