For years and years I collected broken and left-over tiles because I wanted to make a mosaic tabletop sometime. I eventually forgot about my collection of mosaic pieces after I had shoved them into a cupboard and firmly shut the door on them. When redecorating our house I had to delegate my former desk into the garden. And it was then that I remembered my former mosaic plans. With the able help of my partner - who is always great when I need to do the "rougher" part of crafting - I made a lovely new tabletop which now has pride of place in the garden.
My partner built me a simple "frame" into which I was to lay the mosaic. Meanwhile I broke up the tiles into small pieces. (Take an old pillowcase or fabric tote-bag and bash it with a hammer. Or take it outside and hit the walls with the bag. Honestly - not only works but can also have a therapeutic effect on you!) Before setting the mosaic in cement, do lay the pieces on a template! I know - time-consuming and annoying. But it will give you an idea whether you have enough pieces, you can try various designs and you can move them around and experiment with the best fit.
Then set up your mosaic top. I would suggest that you lay the mosaic in stages. I had especially fast-drying cement and had to work really quickly so that the cement didn't set before I had transferred all my mosaic pieces. So cover only a bit of the table at a time with cement - thick enough to stick the pieces in. Don't push them in all the way, let them stick out a bit and create gaps between the individual pieces. That's where you will later spread the grout. Some of your tiles may have been thicker than other, so also make sure that the pieces are all flush, otherwise your tabletop will be uneven!
Once you have arranged all your pieces in the cement, you will have to wait for it to dry. Allow for as long as your cement instructions advise you. Only then mix the grout and fill the gaps with it. Again, let it dry as instructed and then clean off the surplus with a soft, wet sponge.
My tabletop incorporates not only bathroom tiles from my parents' house, but also broken china pieces and some handmade pottery tiles from my partner's aunt which were finally given their proper place.