Start by clicking edit on the publication you want to add or make a shared object from. From here you will then want to go to the editor, clicking the Edit Pencil Icon on the desired page.
Once you are in the editor, Add two objects or select two objects you want to make into a shared object. Once you have this, highlight the two elements by clicking in empty space and dragging over the elements. Then, right click on the elements and select Group, or if you're on a MAC you can use Command + G or if you're on windows then you can use Ctrl + G.
Once you have your group then you will want to right-click on this new group and select Convert to Shared Object. This will then change the selection color to purple, which indicates it is now a shared object.
You will then want to go to another publication to which you want to use this newly made shared object also. Once in the other publication, click the Add button on the left of the editor and at the bottom of this window, you will want to select the publication from the drop-down menu where you created the shared object. Now you will see the list below of shared objects that are inside that selected publication.
Once you have found the created shared object, you can simply click or click and drag the shared object into position on the publication.
You can still edit the shared object from other publications (This will edit the shared object on every publication, regardless of change), by selecting the shared object, then on the right side of the editor under Object Options, you will see a small block of text with a purple Edit button. Clicking this and then make changes to the shared object.
After editing the shared object, In the same area under Object Options, click the purple Leave Edit button. This will then lock the shared object so that it can no longer be edited.
When you visit the publications with the same shared object you will notice that the changes you made previously will have made the same changes across all publications with the shared object.