The drawing at the left shows the basic appearance of one side of the trellis and the positioning of the horizontal cross bars. If you are using natural wood pieces, there's no reason to be precise. Your trellis will look more rustic without exact placement of the parts.
I make a full size template (out of cardboard) of the completed side of the trellis. I then lay the pieces on it and pre-drill holes. This prevents the wood from splitting when I insert the screws. By using the template, I never have to measure and I can make many trellis sides in a short time.
For one trellis you should assemble two complete sides. Once you have the two sides complete, stand them up and lean them together (see small drawing at right). Make sure the feet of the two sides are the same distance apart as they are on the assembled pieces (about 27"). Tie the two sides together at the top to stabilize your construction while you add the cross bars that hold them together.
Look at the small drawing at the right again to see how I place the cross bars. (Here's another good reason to use screws. It's practically impossible to hammer nails into the trellis now without loosening other joints.) Look at the enlargement of the join to see where I place the screws in order to make the trellis sturdy.
Add decorative features as you wish.