Instructions
Go to the SkyBlock download page on the MinecraftMaps.com website (link in Resources).
Click "Download Map" and "Save File."
Launch "Minecraft." On the sign-in screen, click "Options," and then click on the link next to "Game Location on Disk." Note the location of your "Minecraft" saves folder.
Double-click the SkyBlock file that you just downloaded to open it. You'll need an archive program, such as 7-Zip or WinRAR, to open the file (see Resources.)
Extract SkyBlock's contents to your "Minecraft" saves folder. This path is "C:\Users\Username\AppData\Roaming\.minecraft" by default.
Launch "Minecraft" and sign in, then start a single-player game.
Select "SkyBlock" from the Select World list. The game will load with the SkyBlock mod.