Install it like any mod pack on technic.
- Launch your technic launcher (install it if you don't have it yet).
- Login to your launcher and search up "Candy Isle+Extra's".
- Click on the modpack and then the "install" button to install it.
- Wait for it to install completely. When you see the "Play" button, click it.
- If your game successfully launches and you're able to go into your single player world, then you did everything correct.
IMPORTANT NOTES: To get Mermaid tail's mod to work, delete optifine in your technic mods folder (C:\Users\YourUserName\AppData\Roaming.technic\modpacks\candy-isleextras\mods) and edit your skin at (1,0) pixel with these following colors at this website: http://www.planetminecraft.com/mod/1710172-forge-mermaid-tail/. This is only optional though if you're okay with having lag. If not, you can keep optifine, but you will not be able to see the tail from the Mermaid Tail's mod. I used paint (which comes with windows) to edit and see the pixels on my skin.
If you have any other questions, please feel free to ask in the comments.