i used to have them, and from memory one way is injecting them with something (iodine or something) or a process of recreating the drying up of the water, so a very gradual decrease in water levels (over a few months) so they are forced onto land (which needs to be provided in the tank). however this is extremely risky to do in captivity, so it is highly advised against. i too was considering trying it, but it is really hard and often results in death, so it really is better to leave them in water... this was a while ago, so it may be a little off...