PCB tinning is the process of depositing a very thin layer of tin, for the purpose of protecting the copper from oxidation. Oxidation on copper makes it more difficult to solder. Keep in mind, though, that electroless PCB tinning solution is not very similar to plated tin. It does not bond to the copper in the same way, and it will itself eventually oxidize and prevent solder from wetting the pad.
You can get an Electroless tin plating kit from Caswell Plating. Caswell's PCB tinning solution is created from the concentrate immediately before use. You must add sulfuric acid, and distilled water (not filtered water! It must be mineral-free) to the concentrate and activator. The solution has a limited shelf life, so don't make more than your planned needs. It also contains hydrochloric acid, so care should be taken.
Another PCB tinning solution is available at Parts Express or AllElectronics. It is a Datak product called "Tinnit", it comes as a powder, and is made specifically for PCB tinning. You add water to it and get a solution that will last a maximum of 2 months before needing replacement.
If you want to take a chance, there is a home brew formula from the Metal Finishing Guidebook:
All of these products contain thiourea. Thiourea is dangerous. It will hurt your thyroid gland, and given enough exposure can cause liver tumors. You can get it from Science Lab. Look at the price.
The process is simple. Pour the PCB tin plating solution into a tray, and either hold the surface to be plated against the surface of the solution (gloves, please), or just let the board soak, copper side up. Of course, one copper side will be down if the board is double sided. You must not let the copper rest against the pan continuously. The temperature needs to be between 80 and 100° F. When the solution is fresh, in 120 or so seconds it has plated all you need. It will plate all it is going to plate in 10 minutes. The solution is then contained for future use. It will eventually begin to plate black, or not at all, in which case the solution should be replaced. Do not add fresh tinning solution to old solution. The chance of contamination is too high.
Use a separate tray for tinning PCBs. Keep it very clean. The electroless tin plating process can be easily contaminated. The thickness of the tin coating will not be more than 0.0005 inches because the solution only tin plates other metals - not tin, so there is no buildup to speak of.
In PC board fabrication shops, if they use metal resist, the tin is electroplated onto the copper. A simplified description of that whole process:
Tinning your home made PCB is a nice finishing touch.