The BEST (not always practiced though) way to wash a boiler out it to let the fire die a natural death and let the pressure deplete itself. Then drain the water, pull the plugs, wash it out with hot water, put the plugs in, fill the boiler with hot water, put a fire in it and steam it up.
This usually takes about 2-3 days if you let it cool down completely. We usually had to short cut a bit - dump the fire in the ashpit, crack the monkey tail on the dome a bit and by morning it will be dead. We didn't have a boiler plant (most tourist lines don't anymore) so we had to do the wash with cold Chama River water. In steaming up, we'd use another engine for steam and fill the boiler of the dead engine using its own injectors, filling the boiler with 180 degree water, then light it off. With the boiler already full of hot water, steam can be raised safely in a couple hours. If you start with a boiler full of cold water you need to add a couple more hours to that.
If you're in a hurry you can blow it out, wash it and steam it up overnight, the old gals don't like that. It's a good way to end up with leaky flues and broken staybolts.