If the mosaic tiles are thinner than the field tiles, then all you have to do is put the thinset adhesive more thickly under the mosaic than you do under the field. Trying to build up the field tiles will result in a mess and an uneven floor surface. When setting the thinner mosaic tile, use a straightedge to make sure the top surface of the mosaic tile is in the same plane as the field tile. But the hassle can be avoided if you work with tiles that are all the same thickness.
Before you permanently set the tile into the thinset adhesive, do a dry run to figure out where the field tiles need to be cut. The mosaic patterns you are talking about almost always come glued together on a fiberglass sheet, which makes it a lot easier to plan those cuts. Start by laying all of the field tiles in place using proper grout spacing but without any adhesive. Then place the sheets of mosaic tile on top of the field tiles in the spots where you want the mosaic pattern to go. Next, trace the outlines of the mosaic pattern onto the field tile using a fine-tipped permanent marker. Be sure that the line you trace is off set from the edge of the mosaic tiles by the same width as the grout lines separating the field tiles.
After tracing the pattern, it's time to cut the field tile. The traced line will probably be irregular, meaning you can use any of a number of tools to cut along the curved, angled or irregular line. A diamond wet saw, an angle grinder with an abrasive wheel, a hand nipping tool, or even a crude carbide rod saw will do the trick. Your patience and skill at this point will determine the final look. Cut along that line very carefully, removing just the line of the marker and no more.
To achieve professional results as you install ceramic tile flooring, there are several things to remember. The floor should be as solid as possible, with no spring to it. I recommend using a decoupling membrane so the tile can move independently of the floor. These plastic membranes look like the bottom of a waffle iron. It's an extra step in the process, but well worth it if you want your tile floor to be crack free for generations.
Be sure you try to get the floor in the same plane. This doesn't mean the floor has to be level, although that would be nice. To be in the same plane means there are no humps, no bumps and no gaps under a long straightedge when placed on the floor. Use a self-leveling compound on the floor to help get it smooth before you start laying the tile.