The electrics forum might be a better place for this, if you ask a mod nicely they should be able to move it for you. I think the way to do it is to click on the "Alert Moderators" link and ask, perhaps someone will correct me if that's wrong.
Bear in mind that this will be notifiable work (in England or Wales, not sure about Scotland & NI). It also needs properly designing and testing, which requires knowledge and test gear that few DIYers will have. For example, what earthing system do you have, and how do you propose to provide an earth when using the genny ?
Apart from that, installation is actually quite simple. L&N from the mains goes to one input on the switch; L&N from the genny goes to the other input; and the output of the switch goes to L&N in your CU.
Personally I wouldn't bother splitting circuits - why complicate things ? Just bung in a 32A intake, you'll soon learn to manage demand - be careful when cooking the sunday dinner if the cooker is electric, and don't try running lots of electric heating or the garage welder.
What you MUST NOT do is what it sounds like you were thinking of doing - and backfeed the house through the garage supply. If you do that, then you cannot have a changeover switch, and you cannot avoid the risk of having a) plugs with live pins and/or b) trying to backfeed the local grid which (as Furball points out) is highly dangerous (and in cases, fatal) to people working on the supplies.
The common way of doing it is with a lead with two plugs on - one fits the genny, the other fits a socket in the house. These are called (for good reason) "widow makers" because they allow for both the hazards I've just mentioned.
A changeover switch (if properly installed) prevents both problems as the generator and mains supply cannot be connected either to each other, or to the house at the same time.