Andrew Moore wrote
I've just ordered some materials from a local timber merchant to build
a small gapped fence.
The wood will be pre-cut to the correct lengths. The slats are 6 1/2
feet high and they recommended 3 cross pieces which will be 6 foot
wide.

I'm ok on putting up the fence posts as I've done this a few times
before with paneled fencing, but does anyone have any recommendations
on making up the fence. I have visions of it all being wonky!

Do you attach the cross pieces to the posts and then add the slats or
would you make the slats and cross pieces in a panel like structure
and then attach them to the posts?

I assume you're using timber posts. The traditional way of doing it was to cut
slots in the posts and shape the ends of the rails to go into the slots. This
meant you had to slot the rails in when you were erecting the posts. Nowadays
you can buy special galvanised arris rail clips that you nail to the rail and
the posts, so you can install the rails after the posts have set. Screwfix
supply them http://tinyurl.com/27o8q

Should I screw the slats to the cross pieces or will nailing them be
fine?

Galvanised nails, preferably 2 at each point. If you can get the grooved sort
so much the better. http://tinyurl.com/2jjed

Also what's the best way to attach the panels to the posts.

See above

HTH
Peter