How to Flush Main lines/returns

The ideal solution would be valves on both ends of the wet return, so a hose could be connected to blow any debris out. In your situation, the only easy valve to install would be where the plug is next to the Hartford loop.
Use a full port ball valve there to avoid plugging up a dinky little valve.
If you discover you have a liking for piping, I would move the makeup water inlet from the Hartford loop to the wet return to avoid shocking a hot boiler with cold water. In the meantime, throttling the makeup water inlet down would be adequate.—NBC edit-something look back to front with the return piping/Hartford loop, and I’ll have to have a look with my good glasses!

You really never need to flush the mains out. They only see steam and a little condensate so there won't be any debris in there.
You have an auto fill on the boiler son you shouldn't have to add water. That device (black box with the digital numbers will add water when needed) If for some reason you need to manually add water, use the yellow handled valve on the lower left, next to the auto feeder. That will bypass the auto feeder and add water. Unless you have a valve or plug on the far end of the wet return (along the floor, you really can't flush the wet return.

Completely agree with @Fred -- there is no need to flush the mains out, nor the dry returns. The wet returns, they can and perhaps should be flushed -- if you are having problems with slow return water.

But I think that that feeder does have a manual feed button...

@Neild5 , if he has a spigot at the other end of that wet return, the valve on the Hartford loop would allow him to close it when he flushes the wet return without blowing crud into the boiler. Some also put a valve on the Hartford loop so that they can isolate the boiler (if there are valves on each main) so that some pressure can be built to blow the boiler out without damaging the main or radiator vents. If the homeowner is concerned that kids or someone might close the valve, he could take the handle off and hang it somewhere in the area.

Your wet returns only amount to the pipe lying on the floor, looks to be less than 6' between the left and right dry return drops.
IIWM, first shut off the power, I would drain the wet return with the hose connection.
Then remove the plug under the large yellow ball valve.
I would remove the small hose drain you just used and put a nipple and cap there.
Where that front plug was I would put a full port ball valve with a garden hose adaptor with brass cap.

Then with drain hose on the new valve I would use the manual bypass fill ball valve to raise water in the wet returns (and boiler) above the sight glass top and then let it drain. Back washing this back and forth until you get clean water out the hose.

Then I would drain the boiler thru the right side boiler drain, as some sludge may have washed over the HL into the boiler, add water thru the bypass ball valve and back wash everything again until clean water flows from both drains.

Important..................Then fill the boiler to the correct level, turn the power on and bring the water to a complete boil to remove oxygen.

By using the manual bypass fill valve for this you do not add any gallons to the readout on your water meter. You are at zero now and any future readings will be water lost somewhere else as the loss should be very minimal.

While it is drained you might consider the full port ball valve on the boiler drain. All these threaded fittings will become more difficult to unscrew as time passes. That explains why a nipple/cap is preferred over plugs.
If you remove it and look at the opening size you will realize how slow the flow would be thru that port.
The larger port will flow more sediment out with the increased volume/velocity.

The large yellow ball valve, if off would not allow water to return to the boiler. It would stack up in the returns and your water feeder would keep adding water.
If you had a drain hose connected then the returning condensate with steam pressure behind it would flush out the returns. I would leave it open until you wanted to do that, then I would remove the handle and hang it on a nail near the boiler.