With great difficulty and patience
It's mostly about balancing it right. So far, I've done walking and falling over which have become difficult. The bit at the beginning of the 6th short where Jake walks across the screen took about 30 mins to photograph. At least. Plasticine when not dried out is a little sticky, and I've used this to 'stick' the characters to the ground.
When falling over backwards, in some cases, there is support behind them. Most times, it's just a firm pressing into the ground, and they stay held up at an angle. Sometimes.
It's things like that which are the tedious bits of stop-motion animation. It goes wrong. Honestly. It does.