This is the kit I use to renumber a loco. Normally it isn't as neat as this :-))
Going clockwise from the right: Fowler Locomotives by Brian Haresnape. The loco at the bottom of the dust cover is the subject I am planning on modeling. The white lid to the bottom left is from one of the girls baby jelly jars. 5mls of water goes nicely into this (using a syringe) and it is here the transfers are soaked prior to affixing to the model. Swiss Army pocket knife. I use the scissors (and although it is currently MIA, the toothpick is handy for poking transfers around on models to position them). Small (00) paint brush. Bamboo kebab stick. Bottle of nail polish remover (Coles Persona for Australians reading this - non-acetone variety). Bachfar Jinty purchased from Ebay (needs fettling to get it to work properly but was purchased with a view to putting a 2mm chassis under it from Fencehouses) and under the Jinty some Fox 10inch numbers (FRH 2004) and some small Late Crest transfers from Modelmaster via the NGS shop (can't tell you the code for those).
The finished job. Took about 30 mins from start to finish. If there are any dramas with taking paint off this is easily fixed with a bit of Humbrol No 33 (Matt Black). Obviously doing something like a Jinty or 4F (which are unlined) is a good place to start as any mistakes are more easily rectified. With a bit of practice and confidence... See Victoria earlier on this month.