No 1 priority is to season the wood or the slices will split and the bark will likely part company.
Slice them the thickness you want....say 5mm or whatever you had in mind and then I would seal the whole thing including bark with a watered down solution of pva (say 80/20 pva/water. Then leave them to air dry in a cold environment that's out of the rain eg outbuilding or garage etc. They must have air circulate all round or they will grow mould. The pva should slow down the moisture loss and also help to glue the bark in place.
After something like 3 months, take a bit of 240g sandpaper to them to bring the grain back to where you want it and then lightly oil (Linseed/tung/Danish it doesn't really matter which or seal with cellulose sanding sealer and either leave at that or further finish if you want a more glossy finish.
Make many more than you want as you're bound to lose some to the drying process.
