Hi David,
I have made a start, but predictably confused myself and stalled

I'm following your instructions and the diagram you linked to. But its when I also look back to your description of how it operates I get confused.
If I have this right we have an output to the camera's USB. An input from my flash trigger. And a 5v (give or take) power supply.
The USB connection to the camera gives a permanent 5v supply, until the push button is hit which interrupts and starts the camera sequence (equivalent to half press). An LED illuminates to show this has happened and there is a window within which we can trigger the shutter. Presumably the USB output has gone back to 5v and then (within the window) the flash trigger goes off which results in the 5v being interrupted again and the camera shutter operates.
So, if I have that right here's the bits I don't understand:
The diagram has two switches - is one the flash trigger input?
It also has two LEDs what's the second one for?
It has only two transistors but your description lists three.
So not quite getting how your instructions relate to the diagram I started with your instructions which I did find easy to follow.
Connect normally-open switch from left transistor base to ground.
Connect right transistor base to collector of third transistor.
Connect third transistor base to ground via 1K8 resistor.
Connect third transistor emitter to ground.
Connect third transistor base to V+ via a normally-open switch and 2K7 resistor.
I got to the last point, but that has the third transistor base connected to + and -. Is that right?
And then you don't describe all the connections etc.
Any chance you can interpret it differently?
I have 3 transistors, each with a C, B and E connection. I get that.
One push button switch
One LED
One input (flash trigger)
One output (USB)
One power supply
And several resistors.
What I think I would understand is if you extended your instructions based on those components. I can then put it onto stripborad fairly easily I imagine (assuming it doesn't matter the order the components arrive at the + or - rails/strips).
I did warn you my electronics knowledge was limited, but I try to learn quickly. Hope your not regretting helping

Chris