Hi there, welcome aboard.
Potions are pretty much the same as enchanted items, which are pretty much the same as spells. There are only some minor differences.
For potions and enchanted items, you get a base power equal to your lore rating. So if your lore rating is 4, your potion or enchanted item can have a power rating of up to 4. Now let's say you want to bottle up an attack spell. For a 4 shift spell, that means it counts as a weapon:4. And that's pretty much it in a nutshell.
Now since it is an item and not a spell you are using, you need to decide which skill would be used. Most likely in this case, you'd use weapon, as it is thrown like a grenade.
For other spells, it works as well. Want to set up an aspect as a maneuver spell? That's what you do. 3 shifts for a fragile aspect, 4 shifts for a lasting aspect.
Now potions can do a few things that enchanted items can't, but they also have a drawback. That drawback is that once a potion is used up, it's gone, you can't use it again until you create another one. Enchanted items can still be used for 1 shift of mental stress, even after their number of uses per session is gone.
On the plus side, you get to give a potion to someone else so they can use them, which enchanted items can only do with a drawback of their own. Secondly, you can increase a potion's power on the fly by putting in some aspects for a +2 to power, so you are not limited to your lore.
When it comes to duration, it's not really a "best before" kind of thing, but about how long the effect lasts once the potion is used. That highly depends on the kind of effect. For example a bottled attack spell would be instantaneous, it happens, it's gone. An aspect would probably last a whole scene by default. Or even longer, depending on what exactly it is.
If you want to add duration, you add the duration based on the time scale (YS315). You need to dedicate a number of shifts of the spells power equal to the number of rows you want to climb up the ladder. So for example if you decide that a spell would last 15 minutes per default and you want it to last an hour, you'd need to dedicate 2 shifts of power to duration. With a Lore of 4, that leaves 2 shifts of power for an effect.
And lastly, remember that potions don't necessarily need to be potions. The name is chosen for all one use magical items. Harry's sunlight in a handkerchief, for example, would count as a potion, not an enchanted item. You could, for example, have a spell carved into a stick and break it to release the effect or anything else you can think of, really.
I hope that helps a bit. If something is still unclear, I'll gladly try to clarify.
