You can lay travertine paving / outdoor tiles in the same way you would install any natural stone or porcelain products. Basically they should be laid on a full wet bed of mortar over a compacted hardcore base deep enough for the site conditions. For further reading please follow the link to our tech guide:
We would additionally always recommend using a slurry primer when installing Travertine paving to help bond the slabs and the sub-base together. This will result in a greatly improved installation.