As far as I'm aware the principle is the same for all combinations, two attachments at the front top and bottom and two at the rear top and bottom and they all need to be firmly attached and fully adjustable, in/out, up/down and sideways - and on both the bike and sidecar. Also the steering needs dampening. There's any number of variations how to do it, but all follow the same principle. The easy bit is fitting the bits, the hard part is getting the alignment right.
I drilled the tool box mounting bracket holes bigger on my A10 to take the top rear eye bolt. When I fitted a double adult to my A10 in the 60s I was able to use the purpose built attachment point on the A10s frame under the front of the dual seat, just behind the petrol tank. On my current A10 it was, and still is, covered by a penny washer held in place by the battery carrier bolt, but the correct fitting isn't available now. Even Ben at Watsonian wasn't aware there was an attachment point there on the A10 (it's probably on the A7 also - for certain year models).
Rog.
(Edit) Apparently Watsonian supply a fitting kit for the new BSA Gold Star. Quote: The BSA Gold star fitting kit costs £495 including VAT and prices for the Grand Prix sidecar start at £7,195.