How would a Player go about Brewing their own Potion or making a Scroll?

I have to do a (hopefully) quick tangent here.

Magic spells have a built-in resource management. Not all characters can use magic (in general) and/or there is a limitation attached on how often or well you can use a spell. Potions and spells counteract this. Scrolls allows a magic user to preserve spell slots for example or prepare other spells. Potions on the other hand allow non-magic user to benefit from magical effects.

So one has to always keep this in consideration when trying to giving access to certain effects, be it in a scroll or a potion. There's a reason why a "simple" healing potion is 50g. It shouldn't be easy to gain access to a spells effect in masses. Of course this is somewhat counteracted by systems, that keep tight track of their inventory (such as most OSR systems).

Okay tangent over.

When it comes to potions, I like the approach that Zelda: Breath of the Wild has with their Elixirs. Certain parts of creatures can be used to cook into a potion that then gives an effect. Most often associated to what the creature is known for. So a Frostbeetle would give you resistance to cold for example. A great inspirational source is for example the Monster Overhaul.

Considering the above tangent, you should expect resources or creatures that are hard to get. So an effort has to be paid. So normally I would reserve potion making to some very specific situation and not directly allow creating "common" potions.

I also award player ingenuity on what they want to use and what they expect for an effect. For example if a player would want to use some "weed" to create a Potion of Flying because it makes you so high, that you think you can fly, then I'm all for it. Throw in a potential side-effect (you will get the munchies after all) and you are all set.

For scrolls you should also incur some additional cost for creating them. At least the required additional requirements (such as diamonds for Revivify). As a fun exercise you can let the player draw the scroll. But they are not allowed to use any letters or such. So once they want to cast the spell on the scroll they have to recall what exactly the spells' effect are.

The other component that is crucial in creation is time. For this you can use our old friend the progress track. Make it harder or easier depending on the effect. Let them describe what they do in every step, then let them roll to see if they succeed in what they wanted to do. Attach a clock to the track and fill it every time they fail a roll.

Do not let the process fail completely, especially if they expended considerable resources to create the potion or scroll. Let them have the item, but attach a side-effect to it. So it becomes a risk-reward scenario.