A gel extraction is used to select a fragment of DNA of a specific length out of a solution composed of different fragments (ideally the difference in length between the wanted fragment and the closest-sized fragment should be more than 200bp). These fragments are often obtained after a digestion .

Start with a lot of DNA (at least 2 ug of DNA to get reasonable yields), and load a gel

We used Macherey-Nagel's "Nucleospin® Gel and PCR clean-up" kit. Its manual can be found here: [http://www.mn-net.com/Portals/8/attachments/Redakteure_Bio/Protocols/DNA%20clean-up/UM_PCRcleanup_Gelex_NSGelPCR.pdf Gel and PCR clean-up Manual]