It is known (there are many papers in scientific journals) that during meditation energy of low frequency components is becoming greater and energy of middle frequency components is becoming lower. To be more precise,  theta waves (4-7Hz) and for meditators with more experience delta waves (up to 4Hz) are increasing, and alpha waves (8-12Hz) are decreasing. 

Also, many studies show that meditation is increasing coherence between left (often considered as logical one) and right (often considered as intuitive one) brain hemisphere (brain waves from left and right side look significantly more similar during meditation than in non-meditative state).

The idea of using EEG in meditation is very simple. You put three/four (depending on what kind of measurement would you like to do) electrodes on your head (again, depending on what kind of measurement you would like to do, you chose the electrode locations). Electrodes are connected with EEG which is connected with your laptop (USB port). On the laptop screen you can see your brain waves (in time domain - you see how their amplitude in microVolts is changing over time), you can see frequency spectrum of your brain waves (so you can see what are the most dominant frequencies at every moment) and you can see the combination: time-frequency chart that shows how your brain frequencies are changing - you should see how your theta and delta waves are increasing and how your alpha waves are decreasing during your meditation. 

An another EEG mode would be to calculate coherence between left and right hemisphere and display the percentage value depending on time: it should increase during the meditation process.

I would to hear from you if you would be interested in such system (I am estimating price on around 300$ with all included: EEG device, software, cables, electrodes, gel), also I would be more than happy to hear any critics or idea on some other ways to use EEG in meditation.