A common way to isolate the carbon is to carefully burn a piece of the wood and use the carbon dioxide given off.
The carbon dioxide is separated out from the other gases.
A common school experiment is to find the half-life of an isotope called protactinium-234m.
It's useful because it has a half-life of the order of a minute and a pure sample can be prepared simply by shaking a bottle of liquid.
Remember that the carbon-14 decays all the time whether the thing's alive or not.We can plot a graph of radioactivity against time for our sample that had a half-life of 10 years.We can use our graph to show that it always takes 10 years for the radioactivity to drop by a half regardless of where you are on the graph.We can then compare it with the radioactivity of the same amount of carbon dioxide from the atmosphere. Even this kind of carbon dating can only be used to date things that were once alive and died less than about 60 000 years ago.Other radio-dating techniques are used to date ancient rocks.