First of all, you must have a soundcard installed in your computer. Then you need some spectrum analysis software, to record and view the radio signals from the antenna. For this task you have a selection of several programs, available free for download. One of the most used is GRAM and Spectran. A quick search on the web, will give you several URL's for download of the software. ( No one mentioned, no one favoured )

Install the software. Read the instructions. Now you have a very simple yet sensitive VLF receiver in your computer. Open up and run your soundcard software. Choose mic input, experiment later with the gain boost select. Open up and run the analysis tool, you selected to download and install.
If you use one of the GRAM versions ( the newest version of GRAM has time- limited operation before registration )
Go to the file menu:
select Scan Input or use (F3), at the Scan Input menu,
first you go to the Sample Charactheristics and select: 44 kHz sample rate,
then go to Display Characteristics and select: Display type / Scroll, Scale dB / 90, Palette / CB, Scroll Mem / On, Time Scale (msec) / 10
then you go to Frequency Analysis and select: Freq Scal / Linear, FFT Size (Points) 1024, keep the Freq Resolution (Hz) at 43.1, Band (Hz) move with left mouse button to read 11020 - 22045 ( wich is Hz ), Spectrum average / 1, PItch Detector / Off.
If you would like to save the transmission direct to your HD, select: Recording Enable / On
then press OK.

Now you have GRAM up and running, giving you a visual display of a 1/3 of the VLF band, from 11.020 kHz to 22.045 kHz. But as the input is yet unconnected, you should see nothing, but some internal noise from the soundcard. Now it is the time of truth, - take the coax from your ordinary HF antenna, preferably an vertical antenna, - or any kind of "longwire" construction, and hook up the antenna via a suitable adaptor to the mic- input of your soundcard.
 
VLF monitors in Europa and US east coast will now see spectral lines from FSK transmissions at 16 kHz (GBR in UK ) 16.4 kHz ( JXN, NATO COMSUBLANT in Norway ) 18.1 kHz ( RDL CIS MIL at hh:28-38-48-58, often with the CIS version of "EAM's" / Strategic Code Messages, using morse in between) 18.2 kHz VTX3 Indian Navy with A1A morse and 20.9 kHz ( HWU, French Navy ) In other places on the Globe there will be other stations like NWC in Australia at 19.8 kHz and 3SB in China at 20.6 kHz. Also more sporadic ICV NATO at 20.27 kHz from Isola di Tavolara at Sardegna in Italia. And you will "receive" a lot of sferics, -  and if you are in a "manmade radio quiet" location, also other natural radio phenomena. And of course the 15.625 kHz spectral line from both your own and the neighbourhoods TV sets.

If you are suffering from strong electrostatic fields, blanking the input, try an capacitor in series with the antenna lead. If you are suffering interference or intermodulation from strong or nearby BC stations, try an inductor in series with the antenna lead ( if you like, -  up to 10 mH ! )

Remember that outside mounted antennas, might pick up VERY HIGH VOLTAGES, capable of killing both you, - and your electronic equipment, during all stages of a thunderstorm. So for your own safety take precautions to protect yourself, your computer and soundcard interface.

Play around with the settings on both your analysis tool and your soundcard, to get the best performance out of this simple, yet effective VLF receiver.

VLF radio signals are following the earth - ionosphere waveguide and are not limited by the "normal" propagation conditions. As always, I have right now, one of these "receivers" up and running with an 8 meter vertical, 8 meters above the roof. On the GRAM display is the weak, but visible spectral lines of the MSK transmission from NWC US NAVY NCS Harold E. Holt at Exmouth in Australia, a distance of impressing 12700 km.

AN UPDATE, TO THIS INITIAL DESCRIPTION, OF A SOFT VLF RECEIVER

It has come to my attention, that several “lowfrequenteers”, has experienced problems to find suitable and not time-limited versions of sound-analysis software.

Wolf DL4YHF, the creator of Spectrum Lab, has June 15th 2001, updated his latest release ( v1.65 ) of this highly recommendable software for radio use. This most interesting software program, has even a special setting for use of the analysis tool, as a "Soft RX" for VLF use. There is even a pre-programmed setting for SAQ, Grimeton Radio, as well as other VLF and LF stations.

Go to Wolf's homepage for details and download of the program: www.qsl.net/dl4yhf with a US mirror site at: http://members.aol.com/dl4yhf
Read also Wolf's own article " Using a PC with soundcard, as a VLF receiver" at: http://www.qsl.net/dl4yhf/vlf_rcvr.html