I have recently purchased a GWS ICS-300 speed controller and it came with a heatsink. I am not sure how to install it.

You take off the sticky on the back of the heat sink, and slap the heat sink on the flat side (where you can't see the electronics) of the ESC.

Then you take a roll of duct tape, roll it around the ESC and heat sink combo about thirty times, take some electricians tape and fasten that onto the end of the duct tape. That should do ya.

Seriously though. The first paragraph is true. I don't use my heat sink with my 300 GWS. Since it just adds more weight to my slowstick.

Cut the clear shrinkwrap and remove it. If you have some on hand or don't mind a Radio Shack trip, dab a little heat sink compound (aka thermal transfer grease, depends on who you ask, either name will get strange looks from different people it seems) on top of the now exposed transistor (main flat, square black and silver component) and place the heatsink on top of it. Use the included clear, square plastic tube of shrinkwrap (I had no idea what that was at first myself!) to re-cover the unit. After shrinking, I poked small (1mm) holes in the top center of the newly installed shrinkwrap over the fins for a little extra breathing. It's pretty tough material and doesn't appear to compromise its overall covering strength.

HTH!
Dave