I've been searching for a way to implement this and have finally found the answer! I can't seem to post links/pictures here though so we'll just have to make do with text.

First off, please be careful as to WHY you're choosing average costing instead of perpetual costing. I've found Brett Hydeman's video to be the clearest and most informative of the lot out there so it's worth the 80 minutes (or 40 mins if you're watching at 2x speed) to find out exactly how Odoo calculates costing.

1) Install Hibou's stock_landed_costs_average module

2) In Inventory > Configuration > Products > Product Categories, create a new product category with 
Costing Method: Average Costing
Inventory Valuation: Automated
(Note: I personally have created two product categories - One for Landed Costs and one for the product, but that is for my personal filtering and accounting purposes.

If you are not seeing the Inventory Valuation and Account Properties, it's because Odoo has separated Accounting and Invoicing in v11 onwards of the Community Edition

To enable this, enter Debug mode and go to Settings > Users > [Select your user] and Select Show Full Accounting Features under Technical Settings. 

The process after that is similar to that of using Landed Costs module, namely:

·         Inventory > Products > Create a product and assign it to the AVCO, Automated Product Category

·         Inventory > Configuration > Products > Landed Cost Types > Create your landed costs

·         Purchase the product and make sure the PO is validated and the goods have been transferred into your Inventory

·         Inventory > Operations > Landed Costs > Create a landed cost with the Pickings assigned to the warehouse pickings that correspond with that purchase

·         Enter your landed costs under your Cost Lines and Compute to allocate the costs across your purchase, then Validate to make sure it's being updated to your current product costs. 

Going to Inventory > Products > [Select your product] should show an automatically reflected cost of your product based on how you have allocated the Landed costs, be it "Equal", "Quantity" or "Current Cost" split. The same Odoo Inventory video by Brett explains Landed Costs comprehensively at around the 59 minute mark. 

PS: Hibou is AMAZING. They're the only company that I've contacted so far that has offered any help at all even after I'd explained that we are a startup and are unable to afford jumping in to Odoo Enterprise given the number of modules we'd need to purchase. If you ever need help or consultation work I'd definitely recommend Kylie and their team!

Note that I'm not affiliated with Hibou in any way, shape or form. I'm just really grateful for their help and would like to give back by spreading the word.