Well, it's not clearly defined what you should do before, but to start the fireman training it's required to make
in a single year approx. 10 days of off-season engine maintenance/repairing/etc. and 10 days during the steam season, helping firemen/drivers in their depot tasks. That as to both have a good idea of what it is about and, for the railway, to see the degree of the volunteer's motivation (a training still costs money and worse, other volunteer's time so the railway might not be willing to train an unmotivated volunteer). Then you're on for the fireman training, both theory, the engines, the line, railway rules®ulations (easier for already mainline drivers) and practice. There it then depends a lot both on the free time one wants to invest on it and how the training goes. Can go from a couple of years to more. Obviously keeping in mind that we're talking of something done at best a handful of days per month.
At the moment I'm at 5-7 days of volunteering per
year. Not willing to put in more for now, I'm happy with that and the railway's perfectly fine with that too (my wife too

).