A six inch wide sander is your best bet, if you want to do it that way. You can use a four inch but you're going to have to move the bridge back and forth some, so that you can take the wood off evenly. On a six inch sander I usually put the top of the bridge facing the direction the sander is going.
Take the wood off the front side of the bridge or the side facing the nut. The finished thickness of the top of the bridge should be about 4.5-5mm thick. The feet should be about 21-22 mm thick. There is a front and a back to the bridge, are you sure your friend did it the right way. If there is a name on the bridge, that is a brand name, such as Aubert that should have been on the back side or tailpiece side. However, sometimes this is done incorrectly. The back side should be straight up and down and the front(fingerboard side) should slant towards the rear especially after you remove the excess wood.
Hold the bridge with your thumb and fourth fingers or thumb and fourth and fifth fingers for stability, and put a little pressure on the back side of the bridge near the top curve with the other hand. Don't attempt to remove your fingerprints with the sander.
Since this is your first time go slow. It's the old, you can take it off, but you can't put it back on law, so be careful.
Since your friend didn't do it right, there are other problems as well such as the thickness of the feet top to bottom and wood that should be removed in other areas, but if you insist on doing it yourself just getting the thicknesses right should help the sound a lot. Do you have a bass player friend who had his/her bridge done by a day by day honest to goodness bass professional? Well, if so, look at their bridge and see how it was done. Maybe they will even loan you the bass so that you can study their bridge while you are doing yours.
Be sure to rub some pencil lead into the groves of the bridge. They are round right? The strings should sit about a third the diameter of the string into the bridge.
A better way is to make a little jig to hold the bridge on a workbench and plane it down with a six inch block plane. Much less dust in the lungs this way, and a much cleaner finish.
Some put a light coating of linseed oil on the bridge to darken it and some put on some clear Watco Danish Oil. Some of the old timers used wax to seal and darken the wood. Make sure you don't get any on the bottom of the feet or they are going to be slipping and sliding around. They do fit perfectly right? If not you are going to damage the top. Another thing, I just thought of...is the bridge the correct width for your bass? Dang, there's a lot more to this than you thought, ain't there.
Of course last resort is to seek out your local qualified bass repair person and get it done right. If the top curve is correct(is it?) and the feet fit perfectly, it won't take him long to do it and it won't cost you more than you can afford.
As we say in old Mexico, buena suerte.
Let me know how it works out.
I saw the Engelhardt-Link economy student cellos are going from just above $700 new, so I think with the 1939 antique Kay authenticity it was a pretty sweet deal. It has a surprisingly rich sound.
