Samsung Galaxy S5: How to Enable and Use Assistant Plus Feature in the Stock Messaging App in Android 5.0.1 Lollipop

Assistant menu is a feature in Samsung Galaxy S5 designed to assist those with dexterity issues or those who want to easily navigate their phones using one hand. This feature also has an option that will let you display contextual menu options when you're using the stock Messaging app so that you can easily access to some frequently used options or features in this app, e.g: compose a new message