How to take dark frames

Dark frames are obtained by covering the telescope and acquiring frames using the same camera settings, exposure time, temperature, etc. that the frames to be calibrated.
If during the night the temperature does not change significantly, I just take a series of 40 to 60 dark frames. If the temperature significantly drops, several dark frames series should be taken. To calibrate each series of frames the the set of darks taken in the nearest time should be used so the temperature of the camera is as close as possible.
In the case of DSLRs and some Digicams, which have mechanical shutter, dark frames can be taken without covering the telescope but making the camera take the dark frame with the shutter closed. You can also do that after each frame took a dark frame and internally calibrate frame with it .

When there are no darks taken with the same exposure time of the frames to be calibrated, some software allow you to use darks taken with exposure times similar "scaled" or "optimized" to minimize the resulting noise. In these cases it is advisable to also take pictures "bias", which are dark frames with almost zero exposure time. "
While this technique is theoretically correct, calibration is much better using dark frames of the same exposure times.