You want to more or less sit it on the susbstrate. You want to keep the rhizome out in the open. You can bury just the roots if you want. Keeping them long may help hold the plant in place until they stick to the substrate.
Whenever I plant anubis in the sub I like to cut the roots to about 1" or so. Cutting them down actually promotes new/better growth. I just took that from what I learned with terrestrial plants. When replanting/re-potting them it is best to cut the roots back for the same reasons.
I weigh my plants down with fishing line and stainless steel hex nuts. The nut can go into the substrate to hide it and the rhizome stays above ground. Good for smaller plants if you have lots of flow.