I think we're supposed to bring seeds of renewal to self-rez. Get them off the ratman spawn near Sutek's place.
I'm pretty sure a Seed of Renewal is just an ingredient for imbuing. I just tested clicking on one and nothing happens, even while standing in the Abyss. Can't plant them in a bowl of soft dirt either.
You can get a seed from the Silver Sapling and plant it where you want to self-rez. But from what I've experienced so far, you can only get one seed per day per character and it only brings you back to the seed-planting spot ONE TIME. (When it does, you just click on your corpse that is on the ground in that spot, and all your possessions whoosh back into your pack.)
I guess if you stick around the Abyss long enough in one day to die more than once, you just have to hoof it out of there for a rez. Have done it enough times now to know that it's definitely possible to get back to your body and retrieve belongings. It just isn't fun. Better to go with company and hope a buddy can rez you and/or keep Sacrifice virtue up to a usable level.
In the Underworld, I noticed that the Ankh room on the three shards that I've checked has two ankhs in it. I assume you can rez at those ankhs but haven't tested it yet. There's also the Font of Fortune at the north end of the Underworld that is supposed to rez and heal you. You just have to hope there are no spiders hanging around when you do. Depending on where you are in the Underworld, it may just be safer to leave via the front entrance and use one of the wandering healers in that big open beach area.