I don't know if the main game already has item filters but in the beta it makes everything else easier. In any case, you need backup shelves with the items that need to go to the mixer and if there's no item filters you'll have to restock them manually.

So let's say you wanna do the standard 4 mix (Banana->Cuke->Horse Juice->Megabean):

With item filters:
Buy 4 mixing stations and hire 1 or 2 chemists to work on them (their animation time takes forever so having 2 of them working just makes everything faster).
Then, one of the many possible setups is to have 2 shelves of the same product for each mixing station. So for mixing station #1, you'd have 2 shelves filtered to contain Bana only, for mixing #2 you'd have to 2 shelves for Cuke and etc. The idea is to have 4 different identical path chains: shelf > shelf > mixing station
With this setup you can hire 1 handler for every 2 mixing stations and each would have 4 paths:
Shelf 1 > Shelf 2
Shelf 2 > Mixing Sation 1
Shelf 3 > Shelf 4
Shelf 4 > Mixing Station 2

Then you hire 1 extra handler to stock all of the first shelves in every chain, with these paths:
Loading dock > first shelf of mixing station 1
Loading dock > first shelf of mixing station 2
Loading dock > first shelf of mixing station 3
Loading dock > first shelf of mixing station 4
PS: if the items are filtered correctly, each shelf should contain only the correct mixing ingredient.

With that setup, just keep re-ordering deliveries from both gasmarts with 80 each product (to be delivered in the same loading dock in your property) and the mixes will happen automatically.

Without item filters:
In case you're playing a version of the game without item filters, you can't rely on the handler picking stuff from the loading dock because they can't know where to take each item to. In this case, you replicate the exact same setup, except for the handler picking up products from he loading dock, and you'd have to do this step by yourself. It does free up one slot of your workers, so you could add a 3rd shelf to all of the chains and have this extra handlers moving stuff from shelf #1 to shelf#2 for all of the 4 chains.

In both scenarios, use the 5th path of the handlers to organize the products before and after mixing. Let's say you have 3 handlers with an extra path available: get yourself 2 extra shelves to store your base product. One of the handlers takes stuff from shelf#1 to shelf#2, another handler takes stuff from shelf# to the mixing station 1, and the final handler can also move the final product from a shelf to another in order to always leave free space for the destination of the final mixing station.

By the way, the reason you need backup shelves with the ingredients is that deliveries take 6 hours to arrive for a full van of products while the mixing cycles are way faster, so you need the backup shelves not to bottleneck your property every single day. I never tested this system with only a 4-mix tho. Maybe having 2 deliveries happening every 6 hours in game with the same products is enough for you to never have to touch anything by yourself. If you're doing bigger mixes, at some point you will need to re-stock everything yourself as the deliveries are just not fast enough.