Dude I would not attempt to try this yourself. I do this shit for a living and boilers can be very dangerous when messed with improperly. That and or you can damage the boiler itself. Not everything is DIY. While the labor shortage is real after hours/emergency calls are still a thing. While costly it’s not nearly as costly as a new boiler.

It’s not rocket science.

Low pressure will prevent proper circulation e.g. you will freeze your ass off.

You need to figure out where there is a connection you can hook the water hose up to?

Once connected the process is as follows:

1. Turn of furnace.
2. Open tap water
3. Open valve on heating system
4. Monitor pressure - don’t overfill it
5. Shut off valve
6. Shut off tap water
7. Turn on furnace.

A few details. Important you follow the order of opening tap water and heating valve as described or you might push water from the heating system into you tap water supply. Don’t contaminate it - so follow the order described.

Do not not overfill… you have a safety release valve which will vent over pressure (looks to be the blue valve on your picture). However … they are rarely used, so once they open? They might not fully close again. Which means they will keep leaking until replaced. So don’t push it.

Hot water expands. Once the cold water you filled into the system heats up? It will expand and increase pressure slightly. So don’t fill it to the max pressure. Once the water heats? It might trigger the safety valve.

In conclusion. Not rocket science. It’s something every homeowners should be able to do on a yearly basis.