Ok someone on the slack community answered this and now its resolved.
Basically, all you have to do is drag and drop one of the elements from the right side onto the layout in the left. Then click on your added element on the left and it will show you two icons on the top of the element. One icon is of the settings and the other is for replacement.
When you click on the replacement icon, a modal would show up and if you have implemented your stuff correctly, you will find your element in the modal.
Also… the tutorial I followed is incomplete as it does not tell you the directory where you should save the icons and images