Turn off the boiler if it is on and allow it to completely cool.
Turn off the fuel valve if the boiler is gas- or oil-powered. The valve is usually on the outside of the boiler.
Lay down a sheet of cardboard on the outside of the boiler to collect any ash that comes out during cleaning.
Wipe down the outside of the boiler with a damp cloth to remove any loose dirt or dust that has accumulated on the exterior since the last cleaning.
Open the service panel at the front of the boiler. You should be able to pull it off, but some models may require you to loosen screws with a screwdriver before removing it.
Suck out any loose dirt inside the boiler with a brush attachment at the end of a shop vac hose.
Scrub the inner chamber with a chimney brush to remove more deposits, and use the shop vac to remove them.
Wipe down the burner, which is usually an elongated tube near the bottom of the boiler, with a damp rag and then rub at it with the chimney brush to remove debris. Allow it to completely dry before using the boiler again.