how to make the subwoofer's AMP?

One can hardly help but note, Bill, that you also neglected to tell the OP how to do what he wants to do... ;)

Another way to go is to search around here for LM3886 -based amplifiers. Bridged or parallel, these are cheap and pretty good amplifiers with a bonus feature of being VERY simple to build.
Hi Cac liu,
try here;
fig 3 shows a 2 pole filter and the rest is all the maths you could ever need to shape your bottom end response.

There is a xover thread running now with boards available, Moxlite has space for 2 bandpass filters (choose from 1, 2, 3 or 4 pole rolloff) and 2 equalisers for other alignments and an input buffer (no ouput buffer). If this is inside your amp box with very short interconnect then an ouput buffer is not required.
It could be converted to give a hi pass output to the satelite and low pass to the bass. In this case keep it inside the satelite amp box.