Hi,

The easiest way to get decent quality gDNA is to buy an extraction kit.  There are lots on the market from most biotech suppliers.  The one we use a lot is the PureLink Genomic Mini Kit from ThermoFisher. 

We tend to use it for cultured cells with good results, but a look at the protocol shows that it will cope with fresh or frozen whole blood and the starting amount needed is 200ul.  See the protocol here:

https://tools.thermofisher.com/content/sfs/manuals/purelink_genomic_man.pdf

We also use Qiagen kits for other applications and they are also very good - to be honest most DNA extraction kits work well, it's more personal preference but any kit like the PureLink that extracts reasonably clean gDNA will give you a good result with Taqman GT assays.

Hope that helps.