1. You're going to have to get the parts to be joined up to red-orange hot - not good if you have anything else attached to the pieces.
2. To get them to this temperature you'll probably have to make some form of makeshift hearth (I use a couple of cement blocks) and use a decent blowtorch to heat the pieces up quickly, otherwise the flux will fail.
3. You'll have to devise some means of holding the pieces together while they're heated - can be tough.
4. You'll need to get some silver solder (getting quite expensive these days for what is, basically, a length of brass-coloured wire!) and the correct flux.
5. Be prepared for some quite tough cleaning after the job's finished - the flux turns into something like glass when it's heated. It'll have to be chipped off, then emery cloth. Don't use acid - it makes things rust unless you're REALLY good at getting rid of it afterwards.
So - solver soldering is a great technique for joining brass and steel - but it takes some practice to make a good joint at the first attempt. I speak from experience - my model steam train has hundreds of silver-soldered joints, including every joint on the 40-lb copper boiler!
If you're talking about a one-off job, I'd try to make a mechanically strong joint in some way, then lock it together with one of the harder forms of soft solder, e.g. plumber's solder. You'll still need the right flux, but both are reasonably cheap from Wickes. Don't use the cored solder used for electronics - it's designed to melt at a low temperature rather than for mechanical strength. Plumber's solder melts at a few hundred degrees, so the metal is barely affected by heat and is really easy to clean afterwards.
(with apologies for the long post...
)