Get the bottom box absolutely level and as close to the wall as possible. The gasket attaches with its own sticky tape to the bottom box. You should not use any washing up liquid at this stage. Once the gasket is in place, you smear a very thin layer of the liquid on top of the gasket. That’s the only place you need it. (I hate to say this, but Richard’s suggestion of putting the washing liquid on the bottom box before the gasket is not correct, or if it is it certainly wasn’t what I was told by Naim when they showed me how to assemble SBLs in my house).
You then apply a continuous bead of silicone to the gasket, and then place the box with the bass speaker on top. Give it a gentle press down, so that the spikes make indentations in the pads, but not so much that they pierce them. If you apply too much silicone it will spread over the edge of the gasket and seal the two boxes together. That is bad. Just enough silicone is the answer.
Then add the top boxes with the tweeters, using the piece of cardboard to get the right distance. You can then play music, there is no need to wait. Check that you have the seal correct by pushing in the bass unit with your fingers. It should come out slowly. If it pops out immediately the seal is no good and you must start again. It’s actually pretty easy if you take your time.