@Giuliana_Conti , welcome to the forum 
For the first part, I think it’s best you have a look at this article in the guide:
For the second layer, have a look at creating dependencies in this article:
I recommend creating dependencies using the timeline view which is visually more rich than creating dependencies within a task’s details pain. Unfortunately, there is no Timeline view in the Project Template editor. So, if you have already built your template, then I suggest you create a project from it, then setup the dependencies using the Timeline view and then save the project as a new template and delete the initial one.
Unfortunately there is only so much that I can elaborate on within a forum post but if you need any further assistance, I am available for consulting and training services, just like all the other Partners in the forum with an orange badge. Feel free to DM me if you’d like to dig deeper.