Plumber's 95-5 soft silver-bearing solder is perfect for this. Pick it up at any hardware store. You can't get lead solder anymore, but that would be fine, too.
Tin both sight and barrel separately as follows:
To 'tin' the barrel, first clean up the steel bright, then apply flux. Heat until solder just starts to melt. Take a small piece of steel wool with some flux in it, and rub the solder around to coat the steel evenly with solder. Apply heat sparingly, just to keep the solder melted.
Tin the bottom of the sight. Small part, watch your heat.
Then clean off both pieces, re-flux, clamp together and heat until solder melts.
You may bang some solder flat into ribbon, then clip some little pieces off and lay them in between sight and barrel. Then you will be able to have the perfect amount of solder without the great big globs to clean off.
Your enemy is too much heat, so heat just enough until the solder melts. More heat will just oxidize the steel, burn the flux, and prevent any flow from happening.