Clean both barb & inside of tube, rough inside of tube with sand paper. Hold the barb(s) with vice grips, or in vice (better) and heat threaded area till hot. Smear all around threads with flux, then tin (coat) with layer of solder. If you use vice grips, have a care not to drip hot solder on kitchen floor or feet.
Now heat tube end(s) of water filled rad, when hot clean inside of tube with flux then coat with solder.
Next insert barb into one of the tubes & heat with torch until solder flows. Then add extra solder at the join seam, a extra inch or so of solder wire should do for this.
When done and well cooled hook up pump and rad in closed loop and run for a couple hours to check on leaks.