Set your UI up so the party list is near the center of your screen. Here's a screenshot of my UI for reference:



Notice how I set the party frame a little off center for ease of access and to keep a tally of everyone's HP without distracting too much from what's happening around me. Normally, you have to click and then cast the spell of your choice. With a macro you simply need to place your mouse cursor over the target's name and then cast the spell. No clicking required. Best bet is to get someone in your FC or LS to join up with you and test it out.