Essentially just a capacitor in line with your speaker, creating a 6db/oct highpass.
http://www.the12volt.com/caraudio/crosscalc.asp#ccc - plug in your speaker's impedance and what crossover frequency you want, and it'll spit out at you what size capacitor you need (C1 is what you want, don't worry about L1)
Depends on the speaker, really. But that would be the general range I'd shoot for. Hell, for no more than caps cost, you can buy a couple and try them out and see which you like best.
you can get a pair for like $10. they do actually work. i used i believe a 250 hz. on my 4x6's one time and they worked well. http://www.crutchfield.com sells them and most audio shops.
LOL ok guy not to be a newb but how the hell do they get wired the tweets have one already it looks like just + to - and the spec sheet said 148-20k hz so i just made it 148 or do you think i should go alittle higher just to be safe i'm doing an install in a company car there will be 4-5 ppl driveing it i just don't want the owner comeing back in a few weeks *****ing you focked up my stereo cause ppl can't keep there hands off of setting