Yes you need a slapper, the letters/numbers (your herd no.) and some ink. You can also buy a pad, which we have - makes re-charging easier, but if you're only doing a few a toothbrush is fine (not hers!). Size is not specified under law.
You can slap earlier, it is a permanent tattoo, but whilst it will be there, may be more difficult to see esp on coloured pigs, once dirt gets applied, so we only slap just before they go, some apply earlier on the gorunds that the letters then grow with the pigs. you pays your money... Also you only slap for slaughter, so if you change you mind and decide to keep a pig for breeding or sell to someone else, you won't want to have slapped it.
Under PRIMO 2007 (2008 in wales) pigs must be slapped on each shoulder.