You are not missing it because it's not there, so this is a "work with your DM" situation.
Lore from previous editions:
Formula from 1e AD&D (MM p. 53):
... the Magic User must hire an alchemist, and the latter will require from 1-4 weeks to create fluids for forming the creature. This will cost 1 pint of the Magic User's blood and 500-2000 gold pieces. The Magic user must then cast a mending spell, a mirror image, and a wizard eye upon the fluid to form the homonculous.
The 1e spells are Magic User spells at 1st, 2nd, and 4th levels respectively. A Magic User had to be at least of 7th level to make a homunculus. (Or have scrolls available). In 5e the spells are a cantrip, a 2nd level spell(wizard/warlock); "Arcane Eye" is a 4th level spell very close to Wizard Eye.
A 3.5e homunculus requires clay, ashes, mandrake root, spring water,
and one pint of creator's blood. Materials cost 50 gp, and a DC 12
Craft (sculpting or pottery) check. There's the added cost (500 gp) to set up the ritual, which requires the spells arcane eye, mirror image, and
mending. The caster/creator must be at least 4th level. (Reference is the
d20SRD for 3.5e, www.d20srd.org under Monsters, Homunculus).
OD&D (Greyhawk, p. 68) had the original in-game form (here summarized): Made and animated through a special formula by an Alchemist and Magic-User. The Alchemist needs 1-4 weeks, 1,000 Gold Pieces, and a pint of the Magic-User's blood. Killing a Homonculous immediately does 2-20 HP damage to the Magic User. (Spelled Homonculous on p. 68, and Homunculus on p. 19, no mention of specific spells).
Game canon/lore in 5e that suggest how to create a homunculus
- It is a construct
- It serves similarly to a familiar conjured with Find Familiar, but is not an independent creature (like an imp, pseudodragon or quasit)
- It is similar to a golem in being inanimate material that becomes
animated (the golem requires an arcane source of magic to make)
- It uses telepathy like a familiar, but the bond seems a lot stronger and the range is greater
- It is related to transmutation magic since material is changed into
something else, permanently.
As a construct (MM p. 7), which is an animated thing created by magic, a logical approach would be to follow the "Create Magic Item" guidance in the DMG (p. 128 to 129).
What level of rarity is a good one for the "How to Make a Homunculus Cookbook?"
- Consider the varying CR (5-16) and cost of making a golem, and the
fact that a manual of golems is "very rare" per the table in the DMG.
- Tie that to a Wizard of the Transumation school being able to create
a transmuter's stone at 6th level.
- Find familiar is a first level conjuration spell that does something
similar by conjuring, not creating, a servant.
The power of the telepathic link with a homunculus informs my estimate that it would need at least a level 6 wizard to make. (Maybe higher, but a case could be made for its rarity to be either higher or lower1). Getting the recipe in a book or scroll (magical or mundane) could involve a quest or an exchange of favors with an NPC wizard.
Another factor pointing toward 6th level was a combination of the above, and the CR(0) of a Homunculus as compared to pseudodragon, imp, quasit, sprite.
- Imp = CR 1
- Quasit = CR 1
- Pseudodragon CR 1/4
- Sprite CR 1/4
- Golems vary in CR: 9, 5, 16, 10 for clay, flesh, iron, stone
Taking an approach closer to 3.5e, you could argue that with enough scrolls, (and the caster successfully making the DC check to read a scroll of a higher level than the caster can usually cast, see DMG p. 200) a wizard of as low as 4th level might be able to create one ... with some risk of failure as the arcane eye scroll is above his normal level (DC check of 10+ spell level would be a DC 14).
Applying the "rule of cool" together, the DM and the player devise a way to make a homunculus in their shared game world.
1 A similar relationship, telepathically, is the combined Pact of the Chain boon for the Warlock, and the Invocation Voice of the Chain Master. The Warlock can get that at third level. It represents an opportunity cost -- the choice of pact and choosing one invocation over many others. It is bestowed by a supernatural patron upon the Warlock.
As noted by J.A. Streich, there will be a spell covering this when XGTE arrives in November of 2017.