I am also still getting acquainted with a new Arca-Swiss 6x9 F-metric camera, although mine has the telescopic optical bench instead of the collapsible one, so my normal set-up procedure is necessarily slightly different from yours. I do have a suggested procedure, however, based on the instruction sheet that came with the "Really Right Stuff" quick-release clamp and plates I purchased a year ago. Just to get the feel of it, I would suggest that you start by removing both the function carriers from the rail so that you have something light and maneuverable in your hands, and don't have to be afraid of dropping or damaging anything. Then verify this for yourself: the rail will only slide into the extension bracket if it is either perfectly level with the bracket, or angled slightly down so that the leading edge descends into the bracket. As soon as you angle it down and below the bracket, the front edge of the rail hits the underside of the bracket groove, and you can't engage the two parts together. So, for the vertical axis, the principle is that the rail should be angled slightly down like an airplane making a landing when it makes contact with the bracket. Secondly, in the horizontal axis, I find it best to pivot the rail very slightly in relation to the bracket. That way you engage the front dovetail of the rail so that it rides underneath the corner of the bracket, and creates a pivot point as you bring the whole rail into alignment with the bracket.

Here's the description from the Really Right Stuff instruction sheet (for plate you would read rail, for clamp you would read bracket):

Position the plate above the clamp, holding it flat (parallel with clamp), with about one quarter of the plate overlapping the end of the clamp. Tilt the _front edge_ of the plate downward, toward the clamp, and engage the front dovetail of the plate so that it rides underneath the _corner_ (only) of the front clamp jaw. To do this, you will have to swing the plate to about a 30 degree forward angle, so that the rear corner of the plate remains _fully clear_ of the clamp. The objective is to fit the front dovetail of the plate underneath the front _corner_ of the clamp, while keeping the rest of the plate entirely clear of any contact with the clamp.

Now--while keeping in contact with the front corner of the clamp--rotate the plate back to a _flat, straight, in-line position_ with respect to the clamp. As you do this, the plate will be naturally forced to "automatically" feed its rear corner underneath the rear clamp jaw. This method of "front-first-feeding" will always work if you maintain contact between the front dovetail of the plate and the front corner of the clamp's jaw. _Let that corner be your "pivot point" to swing the plate's rear dovetail into the clamp._ Last, do verify proper engagement by wobbling the plate about its front-to-rear axis, and then tighten the jaws. Practice makes it all very fast and easy!"

The length of the RRS description just goes to show how hard it is to make a verbal translation of a movement one can accomplish in seconds, but I think you'll find that their fundamental advice is sound. Just think in two axes, vertical and then horizontal, engage a corner, and pivot. Once you've got the movement down, add the function carriers with their format frames back on, move slowly and deliberately, and I think your problem will be solved. Good luck!