Mosaics, You sure about that??
If you must however, As mentioned above try and get some the same thickness as your existing tiles, it's quite difficult without a few tricks of the trade to pack out mosaics to match existing tiling.
They will come on a sheet held together with a mesh, you'll want to try and set them out so that you don't need to cut individual tiles as this is very time consuming and generally looks a bit crap, especially if they're glass mosaics. If you need to cut the sheet, run a Stanley knife through from the rear.
If glass/stone you'll need a white adhesive, pref cement based (I use PCI Nanolight), and use a flat bed so that you don't see the ridge lines through the tiles once dried.
Apply adhesive, lay in a sheet of mosaic and then I use a grout float to pat them in, this spreads the load rather than pushing them on by hand which can make the individual pieces of tile go crooked.
Let them dry out fully before grouting, if their glass they won't suck up any of the moisture out of the adhesive which increases drying time, this is the primary reason for using cement based adhesive as it sets with a chemical reaction rather than drying out.
Use a wide joint grout as std grout can get pulled out of the joint as you was it off leaving the joints not as full as they should be. after washing, leave it to dry overnight then polish with a DRY Microfibre cloth.
Any more questions?