About Crafting Magic Items
TL;DR: Potion of Alter Self is not an official 5e item. You need to first homebrew it, then you can think about crafting it.
The only known restrictions to crafting are mentioned in the section you cite, although it is delegated to DM's choice.
To start, a character must have a formula that describes the construction of the item. The character must also be a spellcaster with spell slots and must be able to cast any spells that the item can produce.
You can decide that certain items also require special materials or locations to be created. For example, a character might need alchemist's supplies to brew a particular potion [...]
Normally, a character who undertakes this activity creates a magic item described in chapter 7, "Treasure."
Therefore, an implicit restriction is that the potion exists and is already listed, or it is a potion that you agree to homebrew. Fortunately, the magic items list is in alphabetical order and can be found in pages 187 and 188, starting with Potion of Animal Friendship and finishing with Potion of Water Breathing. In particular, note that potion of alter self is not a possible 5e magic item, therefore, you will not find any written rules on crafting it, as the item does not exist.
Homebrewing a magic item
The guidelines for homebrewing magic items are described in page 284 of the DMG. There are no specific rules for potions, although potions usually fit well in the "Modifying an item" section. You can find a potion with similar power level (2nd level spell) or similar flavor (Gaseous Form, maybe?) and modify it. However, as you mentioned, this is heavily delegated to DM's discretion and little details are given. In fact, homebrewing of magic items is probably the less detailed one, and even the existing magic items do not follow a good Power Level x Rarity correlation.
You can't follow RAW here because RAW the item you want does not exist. Everything in the DMG about creating new magic items (homebrewing) are only suggestions, not rules, and as such, they are (probably intentionally) vague and left to DM's discretion.
Additional rules to crafting magical items are provided in Xanathar's Guide to Everything at page 129. However, none of them are particular to potions. Instead, they expand on the suggestions on ingredients that will allow the players to craft magic items.