Hiya,
When i used to (create/make/install whatever you like to call it) koi pools i used a basic rule of thumb then a deviation from it
Water rolls downhill obviously (doh lol) so the best natural are to create a pond would be where there's a dip/shallow/natural area that could in theory trap water in nature
Now as humans most backyards are nice n level so it's pleasing on the eye but not so good for natural looking manmade ponds n pools
Just go over the area by eye, try and crouch down and look for dips/sloping parts in the area you can dig
If there's say a 1 way slope then as you dig move the spoil uphill to the higher point to create a kind of hill effect that would trick the brain into thinking the water pool/pond is there as water has trickled downhill and made a pool/pond
Say there's no slope and you're on the flat or worse a bump, don't panic, again it's a matter of moving the spoil to con the eye, move spoil (dirt/soil/rubble) and spread out around the hole especially if you can towards a bush/shrub/hedge etc
In your pics if my eyes are reading it right, from the hedge/tree, you have a left to right sloping down slightly, also from hedge/tree head on slope
So dig away and send your spoil up towards the hedge/tree area so it looks like water running from there has made a dip and over time has eroded away and filled with water, and if you can slope some spoil from the edge of intened shape so the water level when done can still be just below the lowest slope/area of the surrounding garden (hope that makes sense)
You could even do a built-up waterfall cascade coming from the hedge/tree area so again it cons the eye into thinking there's a water source beyond and out of sight
Just look and take your time having ideas
Sand or a line marker for grass can really help you decide, easier to remove than fill a hole back in or alter a hole especially if there's rubble in the soil lol
Tried to explain and hope it might help
Tony