Some generic definitions of “turbo shandy” can be found, as usual, on Urban Dictionary. The variant I'm familiar with is made with Tennents' Super Lager and Smirnoff Ice, with a double vodka added because, well, why not?
PS Anyone who tells you that you should use Irn Bru WKD instead of Smirnoff Ice is a lying, Philistine cunt. And/or Jacko.
PPS I genuinely don't understand why someone with access to Google would instead expend time and effort typing their question into Cunts Corner and then wait hours or days for a response, especially given that said response is usually something along the lines of “Fuck off, bellend!” It seems to happen a lot, though.