Tours are advertised via the Facebook pages (one for the society and another that provides updates for the shed and stock) and booked via Ticket source link as Iskra says.
I understand that there are tickets still available for next weekends Rail rider diesel brake van tour but all other tours this year are sold out. Diesel tours currently are hauled by Hunslet 58 / 402 and steam tours are hauled by Barclay 2320 / NCB no54.
Janus No1 is out of traffic for wheel turning and is expected back soon, meanwhile overhauls continue with Barclay 2369 / NCB no8, Peckett 1438 and Bagnall 8368 Horsa.