Its simple. Once you have activated this plugin so insert either ID of your input field or maybe some class.
e.g.
#datepicker-filed
it means that your input field is having <input id=”datepicker-filed” />
ID like this.
another example is
.datepicker-filed
it means that your input field is having <input class=”datepicker-filed” />
Try and let me know if it works. Your input field will simply start working as a datepicker field on click in it.