Generally accepted is to use a soil/substrate using a high ratio of "mineral" components, again, but not as stringent as for adult plants. A ratio between something 75/25 to 80/20 is mostly advised but it is not an irrevocable law.
The same rules as for adult plants apply. Substrate must be well draining and contain a low amount of "fines" that can clog up the soil and sieving out the dust from the mineral fraction is highly recommended (I use and old flour sieve or a 0.3 mm mesh sieve if the material contains much fine material). However, in comparison with adult plants, this substrate can have some more material to hold/store moisture for a longer time as this is beneficial for the growth of the seedlings, especially for root growth and survival. It is generally agreed upon that seedlings may require more moisture during their "childhood", the caveat being to pinpoint when this "childhood" ends, but it certainly will not be before one year.
All this can be achieved by playing on the amount of organic material or by using a higher ratio of "capillary" stones/material like pumice, vermiculite (small grain), lava/scoria, broken bricks and similar, versus hard stones. My preferred material is black scoria. Other "rocks" are also good additional material: old, weathered granite, aquarium quartz, broken rock (Bonsai).