Very Simple... If you are like me and got the iPhone4 before the jb and still want to use your jb 3GS for MyWi or whatever reason...

All I did was match up the new sim card's microchip over the old sim card's microchip. Use your best judgement while doing this.

There is two ways to go about it. Trace the new smaller sim card with an exacto knife while holding it over the old sim card making sure the microchips still line up. After you get it traced you can attemp to cut out the square that the trace left behind.
or...
line up the microchips and just trace out the top portion and cut off the entire bottom of the old sim card. Place the new smaller sim card flush with the old sim card while in the 3G(s) sim tray. put some tape on the back and you should be good...

if done right your 3G(s) should have service...

i'll try to get some pics up later... and hopefully this is not a repost... if it is... mod's please trash thread