The great thing about finding bones outside is that for the most part, they're already clean! I'm from a small hunting town, going out for hikes to look for bones and sheds is a popular thing to do in the spring. I found bear skull on a game trail once, you don't need to do much more than collect them in a garbage bag or a cardboard box, old bones aren't toxic, just dirty. If you're going to use them for garden decorations then I probably wouldn't do anything to them, just drop them back in the dirt once you get them home, but if you plan on bringing them inside then you'll certainly want to clean them.
Taxidermists have several methods for cleaning a skull, the trickiest part is always de-fleshing (and getting the brains out) which is often done with maggots or beetles, but can also be done by boiling the skull, or just soaking it in water (which is stinky and gross). In your case it looks like nature has already done all the dirty work for you! All I would do no is boil the skull for a bit (outside, some BBQs having heating elements that work good for this) to disinfect it and soften up any grime you may need to scrub off (use a bottle cleaning brush to get up inside the skull and scrub away any leftovers) then proceed to whitening the skull.
Whitening is best done with Hydrogen Peroxide, you can use the over the counter stuff from the drug store, or you can buy the heavy duty stuff from a taxidermist. just soak the skull in a tub overnight or for a day or two (no longer or you can start to ruin the skull) then let it dry for about the same amount of time.
After your skull is nice and white, you can superglue any loose teeth back in place, and finish your skull with a spray coat of clear wood finish.
Of course you could always give your local taxidermist a call to see how much he would charge you to clean them up himself, you'd be surprised how cheap they'll do some jobs, it would probably be good to consult them about the condition of the bone either way.