sundial, compass, spirit level. All-in-one time piece.

Step 4 More Marking Out

The third-of-a-twelfth is an offset for the months, since the solstice occurs about a third of a month before new year's day.  Use this length clockwise from a solstice mark to place the start of January.

Then, starting from January, use the twelfth lengths all the way around the plate to mark all the other months ( you'll get better accuracy if you mark out July as well: divide into six, then divide again)

From these twelve month marks, draw lines from the edges to the centre, but stop when you hit the circle.  The less markings you put in there, the better.

The dome is a little harder, but since it's the same size as the plate, you've already got all of the lengths you'll want to use.
You need a line going all the way from the edge of the dome, over the very top, and back down to the exact opposite side.
You can use the freshly marked base plate as a guide for the edges but the line itself poses a fair problem.  I had some success with using the edge of some sticky tape as a guide. It wasn't perfect, but it was better than free-hand. 
This line will cast a shadow, so make it wide enough to do so - but fine enough to look neat.
[edit] The second image is a template.