Start by going to the article you wish to change permissions for. Click on the green "Edit Article" button on the right hand side.
Now in the editor, click on the "Permissions" tab at the top of the editor page.
Most articles have the same permissions as the category they belong to. That's what is meant by "Inherit Permissions." If you want the permissions to be different than the category (for instance, giving permission to a specific partner to see only one article), uncheck the box next to "Inherit Permissions."
This is the new screen that will pop up. To give permission to a specific person, you will have to look them up in the People tab in TD to find which "Group" they are a part of. Then, give permission to that group.
NOTE: You MUST remove permissions from the group once the purpose for giving permissions has been fulfilled. TO do so, simply recheck the box next to "Inherit Permissions."
Find the correct group in the list. You cannot search for the group, so you will have to scroll down the list.
Once you find the group, check the box next to it and it will be inserted in the "Add Groups" box.
Click "Save," otherwise your changes will be lost.