After the holes are drilled, try cutting a small piece of plastic window screening and put it over the holes, to keep the soil in and let the water out. You can also use a paper towel or a coffee filter.
What to plant
It is important to carefully assess the plant species to be placed inside. In general, they should not be chosen if they are too large or plants with rapid growth. The tea cups are small in diameter and the depth is not excessive. Fast growth or worse, if dimensions are not adequate, the mini garden will quickly fail.
Succulents are very suitable as candidates. Visit your local nursery and you will be sure to find a tiny-sized succulent plant. Other species you may want to consider are very small ivy, violets, ficus or primroses.
To pot them up, part fill the teacup with suitable potting mix, remove the plant from its pot — removing soil and part of the root ball, if needed — position in the cup and backfill with potting mix.
For a more lively display, look for plant species with variegated foliage or choose the bulbs that are perhaps about to bloom soon, such as hyacinths.
Once you've planted your miniature teacup gardens it is very important to water them.
If you let them sit in water, succulents will quickly rot, so use a fast-draining potting mix, like Yates Thrive Cacti & Succulent Potting Mix and ensure your teacups or containers have drainage holes.
— Courtesy of Yates