To get to Tarapaya catch a micro from Potosi; this will cost about $0.50 and will drop you off on the main road. You will see the uphill steps/rocks; follow these to the hot springs which are walled in because they are extremely hot. You can see them bubbling!
The pond is scenic, and there are nice walks around the springs; unless you want to do some real walking half a day would suffice.
To get back to Potosi, go back to where you got off the micro and just wait. They have boards in the window saying where they are heading, but if you don't see it flag it down and ask. You may have to wait a while but one will turn up, and it will be crowded! Be prepared to be squashed and sat on!