It is a challenge but it can be done! Just ignore the rings in the bottom (don't try to line them up) and some bottles will be higher up than others (so the narrower bits of some are next to the fat bits on the others) You need to put the last 2 in together. then put all the hoods and rings on top of the bottles (there is quite a bit of spare height! - as long as the basket thingy fits in ok then it's fine). Just have a play about and see how it goes.
I don't assemble the teats and rings, I just use tonges to drop the sterile ring over the teat and then put my fingers right on the edge of the ring, and use the tongues to put the teat through - again it's tricky but can be done.
Although, as far as I know, it is the heat that sterilises everything, so theoretically it would sterilise just as well if it were assembled.
Hope that all makes sense - i've probably not explained it very well, if you want me to send a pic of the loaded steriliser let me know!
xxx