Here is How to Save your Bonsai.
Place a fine mesh screen or broken pot shard over the drainage whole to minimize any loss of precious soil as your bonsai tree gets settled in.

Layer some small pebbles along the bottom to ensure the best drainage possible.

Then fill the remainder of the pot with the porous soil mixture as the growing medium.

Different types of trees will be happier in different soil compositions but a good rule of thumb when planning how to care for bonsai trees is a mixture of loam, sand, and peat or compost in a ratio that suits your particular tree.