With the larger plants in place, add a little potting mix then position the smaller plants. Again, add only enough potting mix that all the plants will be equal depths when the arrangement is complete. When working with cacti in close proximity, use gloves to prevent needle injuries. After all the plants are in place and you're certain they are equal depths in the soil, carefully fill the container with potting mix. The potting mix should be level or slightly below the level of the root balls. Don't pile potting mix up around the stems.