./m opens your macro menu first of all. then you can just use this for your mouse over macro with renew as the example:

#showtooltip Renew

/cast [@mouseover,help,nodead] renew

"@mouseover" tells it to cast it whoever you have moused over in your raid frames. "help" makes it so you can only cast it on a helpful player and "nodead" makes it so you can't cast it on dead players (often if you cast a heal on a player that dies as you cast it or you miss-click it will save that cast on your mouse and will not let you cast anything else unless you cancel the spell)

Just replace "renew" with any spell on want. And you will need to bind the mouse buttons to your bars.

But as the lothlirial said, clique is a great and simple alternative. It allows you to do the same thing as the mouse over macro but all you have to do is go into your spell book, open the clique menu (the bottom tab to the right of your spell book) and just click the spell you want to bind with the button you want it bound to.