How to refill a single zone, oil fired hot water boiler for baseboard radiant heat

[McG]

Please help me understand this system and how to get it up and running again. I had to lower a pipe, so I drained down the system (don't know if I did this properly) cut the pipe and lowered it. Now I need to get the boiler running again.

I think I need to flush the pipes first to remove any solder/flux, dust from the repair job and to check that my joints are holding. What direction is the water going to flow, which valves to open and which to close. Any help would be greatly appreciated.

Here are a few pictures. I'm not a pro so please do not use pro lingo or abbreviations. Thanks.

The make-up water enters the system from behind the expansion tank.