Start with an order of 0603 resistors off a reel from Digikey (pt # RMCF 1/16 10KFRCT-ND). I have been using the 10K ohm variety with +/- 1% tolerance. With 400 cars I was drawing only about 350 ma, so not much current draw in the grand scheme of things. Next batch will be 7.5K ohm as it seems they are more sensitive for our light cars in N scale, and the overall current draw won't be that much more to the layout even if you have a stable of 500 cars (I am going to guess at around a half amp for my car fleet). I would rate the current R value as good at this time, but there are some cars that get a bit fussy at times. This could also be due to other factors like track or Blackener on the wheels, but I have not followed up yet since low on the priority list.
Here is a quick tutorial on how I do the install, but there are a number of different ways if you peruse the internet for more....
Materials used in the project:
I start by giving the wheels a bath in alcohol to remove any oil residues. Then I use a piece of masking tape doubled over to set the wheels on so they don't move around. If you use a small piece of sheet metal (like a 3X4" piece), you can lift it up to inspect each wheel more easily as you progress than just laying them all out on the workbench. I lay out about 10 to 20 wheels at a time for a production run. Located elsewhere on the bench, I set out another piece of masking tape to catch the resistors as I release them one at a time from the reel tape. This helps to prevent them from getting lost (they are kind of small). I take ACC and apply it directly to the wheel axle adjacent to the insulated section of the wheel with a piece of wire.
I try to coat the entire axle end section back a strong 1/8" from the insulator (where the axle meets the insulated wheel), insulating the metal axle before I set the resistor in place. The resistor has one exposed pad at each end that will cause a direct short if they both are touching the axle. Once that has dried, I pick up a resistor with the tweezers and apply a drop of ACC to the bottom of the resistor with the same piece of wire.
Then I set the resistor in place and wait for it to dry. It should be pushed up tight against the wheel insulator as close to square as possible. Next I apply a small amount of ACC to the sides of where the resistor is now sitting as a preventative measure to keep any conductive paint from sneaking in under the resistor later.
Next I use a smaller piece of wire to apply silver conductive paint to each end of the resistor, forming a bridge at each end where it will conduct to opposing sides of the insulated section.
Above pic shows BLMA wheels in the front, and Fox Valley wheels in the back row. Make sure to get the silver paint to cover over the ACC insulated sections. The price of this silver paint has skyrocketed from $10 to about $50 as of late, so glad Ray stocked us up on a supply while it was still cheap. After that has dried, I check each with an ohm meter to verify the proper resistance value. Of note, some wheels can take awhile for the silver paint to dry, so don't be alarmed if your meter shows the wheels as an open circuit when you first test them. To complete each wheelset, I use a water base black paint to camouflage the use of detected wheelsets.
Do not use oil base as the conductive paint will be compromised. Even as small as these resistors are, I have found there are some cars that the resistor hits on underframe details, but moving the resistive wheel to the opposite side of the truck has resolved any issues so far. I typically place one resistive wheel per car on the inboard side of a truck in each car, in an effort to try to keep resistors spaced out as far as possible throughout the train (to keep the short OS sections reporting). I am running one resistive wheelset in each truck of cars 80' or more, likewise with cabeese, but the later get their wheels installed at the far ends since more critical for accurate location reporting.
Of note, Gorilla glue is an alternative to ACC that others have had good results with...
Hope the tutorial helps, and look forward to seeing some visiting trains on the layout. :)