OK Let's start!
The drawing of Frame #31 directly from the book.
Before doing anything else mark your frames clearly "FRONT" and "REAR".
We will start with the REAR FRAME First. I cut out the frame so that I still have my cutting lines on my drawing, but especially at the bottom I try to get close to the edge of the frame notch. This is so that we can position that side as close as possible to the edge of our working surface.
Here are the two sheets that make up the assembly of Frame #31. I have marked the three parts that will make up the rear frame with the red lines.
BEFORE cutting the frame parts from the sheet, mark the REVERSE Side of the parts that are to be cut, so that you know at all times which are BACK and which are FRONT! This is IMPORTANT!
I use an ordinary X-Acto - type (Chinese brand) knife to remove the frame parts from the sheet. Yes, it does take time. Make sure that you cut from both sides and replace blades as soon as you see that you have lost sharpness.
This is what we then have. Now just sand away the little nibs where the frames were attached to the sheeting - no more.
Which then brings us to the working surface. You say you will have to find a suitable table to use the clamps. I have the same issue, so I use an alternative - use a bread board or cheese-cutting board or any flat piece of wood that is larger than the frame. Even the Leudo's building jig will work for this!
Now simply place your cut-out drawing on the working surface so that the bottom-end (frame-notch end) is closely positioned to the edge. You will see that the Leudo's jig is too small for the assembly of the FRONT frame, but for the REAR frame it is fine.
Now position your centre frame piece so that is it perfectly aligned with the drawing - and CLAMP the sucker!
You will see that the clamp i slightly offset to right (as we look at it). That is done for a specific purpose.
And now you will see that the one arm of the frame lines up perfectly with the centre piece!
Please don't glue anything yet. Maybe you can practice getting that centre notch aligned perfectly with the drawing and clamping it. Now there are no pins that get can get confusing or obscure the view.
Taking into account that you cannot stand unsupported, I forced myself this morning to do all of this while sitting down. It was no problem. (I only stood up to take the pictures).
Good luck and good practicing my friend!