Creating sharepoint 2013 workflow in designer

I am new to topic of creating workflows in Sharepoint. What I would like to achieve is creating a workflow that acts as following:

1. It starts automatically upon adding new item to document library (completed)


2. Task is created where a person fills out required fields that are then copied to corresponding columns in document library


3. Then another task for different person is started in which another set of fields is filled in. Those fields should also appear in document library. On the other hand the form to edit task should display the data that was filled in by a previous person.

I tried to create a workflow in Sharepoint Designer 2013 but I'm having trouble passing data between task in tasks list and item that started the workflow in documents library. Basically what I'm missing is ability to create a task in sharepoint designer workflow where I could fill in custom fields. I created custom content type based on sharepoint 2013 task but the actions Assign a task and Start task process don't allow me to fill in custom fields from my content type.