There are various ways to get the combs started straight.  Some top-bar
beekeepers have beveled the underside of all the bars to a "v" and run wax
along this.  Others use a strip of paper or masonite set in a groove along
the bar.  I built and used a hive in which each top-bar was composed of two
5/8"-wide bars.  A 2" deep strip of duragilt foundation was pinched between
these two "1/2 bars" and they were tacked together with brads.  The
foundation was thereby secured between them.
 
Here's an interesting idea for starters (sorry, but I don't remember where
I saw it first):
 
You melt some beeswax in a tin or other container.  When it is good and
liquid, you drop in a piece of twine or heavy string, which has been cut to
the length of the desired comb.  The string will become soaked through with
hot wax.  Next remove the string, let it drip for a couple seconds, and
immediately apply it along the center line of the top bar.  The wax soon
congeals and cools, forming a definite ridge along the bar, and the bees
consider this the "nub" of a new comb.  They will construct the comb along
this straight guide.  I must mention that I have not tried this method yet,
but will in the spring.  It sounds fairly straightforward and easier than
drizzling wax into a groove or gluing in a masonite strip.