Well, the basic plans are readily available everywhere; your local park (mit ruler unt paper), libraries, online, ...simple as typing "plans picnic table", "plans picnic table free" and variants thereof into your favorite search engine (Instructables excepted though, since it's not nearly qualified to be called a search engine) but as to preparing the sleepers, I think it's simply a matter of re-milling the lumber into the dimensional stock you require, by ripping them with a table saw, then cutting the appropriate lengths per the design and drilling or notching or whatever the design calls for...You SHOULD, of course, be very careful, since there may be (and quite likely WILL be) foreign objects in the sleepers, nails screws, lag bolts, that could at the very least make re-milling them an exciting venture, and at the very worst could severely injure you or anyone who helps you execute the milling operation or damage your saw blade beyond repair.
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Good warning on the foreign objects in the sleepers, seandogue. Also, wood railway sleepers are often impregnated with chemicals (commonly coal tar creosote) to preserve the wood. While they are weather resistant, they may not be suitable for for a table.http://en.wikipedia.org/wiki/Creosote#Health_effects_of_coal_tar_creosote
Nice point. I'd forgotten about the creosote, which aside from the foreign object tissue will gum up the saw blade and put strain on the motor. As the vapor from heating the wood during cutting...Nasty stuff.
After reading that and the difficulty in actually making lumber out of the stuff, I'd find something else to use for the table and benches and use the ties for landscaping timbers.
This design might suit your needs, as it uses the same size pieces throughout. Just slightly alter the design to fit the size of your pieces.