This article describes one easy way to mark the location of the holes for field-frame bar tenons to the field-frame rings. The assumption is that the tenon of the field-frame bars have been cut, and that the holes in the field-frame rings will be drilled and files to shape. An easier alternative - provided proper tools are available - would be to use forging and punching to achieve the same results.
The process step-by-step Edit
First make a steel template which has rectangular holes to mark the location of the field-frame bars:
Use the template to mark the location of the field-frame bars:
Mark the location of the holes with field-frame bar tenons:
Unless you've cut the tenons very carefully to the same width and to the exact center, make sure you the tenons with particular holes.