1) Whilst being logged as a JIRA administrator in your JIRA instance, click on the cog icon (1) in the right upper corner of the screen and select "Projects" (2):
2) From a Project list please access a proper one by clicking on its name:
3) Scroll down a little bit and find "Workflows" (1), then click on pencil icon next to the workflow assigned to a particular project's issue type (2). On a newly loaded page you can then edit a desired workflow's elements.
4) Switch view to 'Diagram" by clicking the diagram button (1) to get a view similar as below:
5) Then, on the diagram select a transition that poses an interest to you to act as a trigger for launching an email. When selected, it'll be highlighted in blue (1). Then click on "Post functions" link in the box on the right hand side (2):
6) Next, as shown below, you need to access "Add post function" link (1):
7) Then please select the adequate post function (1) to be included in a desired transition by pressing "Add" (2) button:
8) Select the created earlier template (1) and click "Add" (2):
9) Make sure that the template is listed in post function list (1), then click "Publish" (2) to add it to the workflow:
10) Email is to be sent when transitioning to the next step (in accordance with the workflow).