collect piece of tissue (e.g., pieces of a leaf or flower) in Eppendorf tube containing 40 ml 0.25 N NaOH
put in boiling H2O for 30 sec (optimum may need to be determined for each type of tissue, for floral tissue 30 sec is fine)
neutralize by adding 40 ml 0.25 N HCl and 20 ml 0.5 M Tris-HCl pH 8.0, 0.25% (v/v) Igepal CA-630
boil for another 2 min
use directly for PCR or store at 4°C for up to several weeks
if stored at 4°C boil again for 2 min before the PCR
We don't have too much experience with this method. Mostly it seems to work though. However, I don't think it is safe enough for critical purposes (e.g., mapping).