For the purpose of this tutorial, I will refer to the main Content Type as Article CT and the referenced content as Related Content CT.
Once you installed the Entity Reference module, add a new field to your Article CT. Go to
Structure – Content Types – Article (or whatever your Content Type is) – Manage Fields – Add New Field – Entity Reference. For the widget, I used autocomplete (but you can choose which one to use). In the field setting, you can select the target Content Type, in this example the Related Content CT.
To test if this is working so far, create a test node of your Related Content CT. Then, create a node of your Article CT. In the edit form of the Article CT you should find your newly added Entity Reference field. Find the Related Content node you just created and then you can save the node. View the Article CT node and you should see a link to the Related Content node you just added to your Article node. In case nothing appears, check to see if the newly added field is set to display in the Full View mode. Go to
Structure – Content Types – Article (or whatever your Content Type is) – Manage Display and you can set it there in case it is not.