To add this new function to your Blogger layout, you will need to do some simple steps to activate it. Once logged in to your Blogger dashboard, go to Layout -> Page Elements and click the “Edit” link for the “Blog Posts” widget. In the pop-up window, you will see the option to add “Reactions” to your posts:
Tick the check box and edit the labels for your buttons by clicking on the link “Edit” nearby if you want. Then finally save this setting.

Adding the code to your template

Unfortunately, setting up this feature doesn’t simple as Blogger comment form, you need to add some lines of code to your template.
If you are using a default Blogger template with no modifications to widgets or the post layout, nothing more to do because the code is added.
If you are using a customized Blogger template, you will need to add the required code to your template manually. In this case, go to Layout -> Edit HTML in your Blogger dashboard. Make sure the check box “Expand widget templates” is checked.