It is not a piece of cake but here is a brief description: L-Edit has a button called "extract". This button starts a procedure which recognizes patterns in your layout and generates spice lines for corresponding circuit elements (transistors, resitors, e.t.c.). For small circuits this works OK. For larger and more compilated circuits you may have to alter the recognision mechanism by adding recognizable patters but this is a big discussion (this is the difficult part). So, to integrate L-Edit and T-Spice you may just click-and-go