How to cast alternate sign on controller

The Medidation Menu is on the Far Right of the Menu.

There are two menus.

One is a menu where you can save game, load game, exit, etc..

The other is where you have your World Map, Inventory and the Meditation Menu. It's on the Far Right of this Menu.

You can see it here in this image:

You can then select the span of time (You can see the current time you're on and select When you want the meditation to end.) and it will fast-forward to that chosen time healing you in the process.

If you're playing in higher difficulties (Any above "Story and Sword") you will not heal during meditation but the game will always warn you about that.

I experimented a bit myself with decoctions, and found the following about the mechanics of them.

They do replenish like potions do, as already stated in another answer. They also have vastly greater durations at around 30 minutes. But the way toxicity works is fundamentally different than with potions. The toxicity from decoctions doesn't seem to decay, it was set to a fixed 80/100 in my case (without any skills in the alchemy tree).

This means that it is impossible to have two decoctions active at once as it would exceed the maximum toxicity value. It also means that while using some potions is still possible in addition to a decoction, it is severely limited. In my case, only potions with 20 toxicity can be used, as that is all that is left to the maximum of 100. And each potion use did cause a signficant health loss as I hit the maximum toxicity value.