Basically you can't do it easily. Unless your Sky box is different to mine the only HD output is the HDMI cable. So you have to use a SCART/RCA adaptor to drive the yellow/red/white composite input. That of course, is SD not HD.
The only other option is to purchase an HDMI to Component box (NOT just a Cable). The main one recoemmened is the HD Fury but that is very expensive. I have had success with this mucgh cheaper one: http://www.hdmiconverterbox.com/hdmi-to-rgb-component-ypbpr-audio-video-converter-1080i.html?___store=default
That allows you to convert the HDMI out from the Slingbox to component video to feed the Slingbox COMPONENT IN and then use the COMPONENT OUT to drive your TV. Although it worked for me on other devices I have never tried it on a Sky box, so cannot guarnatee it will pass-through protected content covered by HDCP.
And remember that if you view your Sky box remotely you will not be getting HD, just a display that depends on the upload speed at your Slingbox location. You will only get near-HD if you are viewing at home on the same LAN.
I've used the simple non-HD solution for years and been very satisfied.
BTW, the HD in the title of the Slingbox simply means you can plug an HD source into the component video sockets. It has nothing to do with the stream quality.