Etched brass kits arrive flat but pretty quickly they need a bit of folding. Bending along half etched lines isn't difficult but it's wise to support the bend. Traditionally you'd use a pair of bending bars but they are more suited to serious metal bashing than effete etched kits.
I prefer a nice square ended pair of pliers for most jobs. The yellow handled pair shown are brilliant as they have relatively long jaws with no wire cutters or serrated faces. You can get right into a corner and grip, then push the metal round with your finger or by pushing it against a bit of wood.
And they only cost me a pound.
Much more expensive is the "hold'n'fold" behind. Parts can be gripped tightly and then bent up by sliding the Stanley blade under the ungripped side of the fold. While not essential, this is a very handy tool for the keen etched kit builder. Long folds, such as solebars channels, can be supported along their length thus avoiding a series of kinks of bends which result in inadequate support.