ideal absolute to relative over an integral basis

[markus endres <me@mendres.org>]

hallo together,

I have the following situation: 

let L/K be a field extension, where K != Q (the rationals).

now, I have an ideal, call it J, in the number field L. J is given
absolute. 
what I need is a relative representation of J over the integral basis of
K. 

is there any simple possibility?

thx in advance

greetings

me