ok - there are a few variants depending on age, if you want/have STB control, but to start you need to do the following :
Put a scart from the VCR to the TV by using the (strangley enough) v-tape and TV sockets. Your video should now work.
Then, connect you SKY box to the AV scart.
Select TV, Menu and go to the input selection (sources) option and press GO
Assign the AV socket a "button" to respond to - this should be SAT for SKY. - toggle up and down and save by pressing go.
You can then assign your DVD player to the decoder scart doing a similar thing - PROBABLY.
On older versions, you use this socket by selecting a TV channel ( nominally 6, but when nalogue goes, use 1) then in the tuning menu, turn AV to ON. The when you select TV 6, the Avant will look at the decoder socket, not its internal tuner.
Ideally make all the connections with the TV unplugged, the turn on and do the menu stuff.
BEST IDEA - download the manuals from this site, for full instructions and set up guides.
Assumes your a paid up member - can't see as I type this.
Good luck.