Heptanoic anhydride or heptanoyl chloride slowly added to test base in pyridine solvent (Pyridine is nasty, use a fume hood). Extract, wash, recrystallize, wash...check purity on HPLC, repeat wash/recrystallization as needed.
Oh yeah, and the anhydride or chloride needs to be in large excess to drive the equilibrium to product side.
Best answer: too complicated just buy test enanthate