Let's assume you have a single product you sell, which you buy at $10 and sell at $25.

• December 31st - you have 573 Units on Hand.
• January - sold 383, purchased 400
• February - sold 402, purchased 450
• March - sold 442, purchased 475
• April - sold 487, purchased 500
  

1. Create and Apply an Inventory Adjustment for 573 Units with the Accounting Date of December 31st - the Scheduled Date you can leave at today.

(If your Inventory Valuation is Automated, you will need to confirm the Valuation Accounts - such as a Clearing Account - on your Inventory Adjustment location for this step)

As the Journal Entry for this adjustment (only with Automated Inventory Valuation) will be posted in December, your Balance Sheet is correct.

2. Enter a single Bulk Purchase Order for January. Set the Order Deadline for January.

• Validate the Receipt.

The Purchase Report will already show this correctly as a January Purchase. The Journal Entry for this purchase will be posted in May, so you will re-class the balances.

• Reverse the Entry to clear out May.
• Duplicate the Entry, post in January.
• Create the Vendor Bill (from the Purchase Order) and date for January.

3. Enter a single Bulk Sales Order for January.

• Validate the Delivery Order. 

The order date will still be May, we will fix this for all Sales Orders after they are all entered. The Journal Entry for this sale will be posted in May, so you will re-class the balances.

• Reverse the Entry to clear out May.
• Duplicate the Entry, post in January.
• Create the Invoice for January Sales (from the Sales Order) and date for January. 

Your Balance Sheet and Income Statement are now correct, since the Journal Entry for the Customer Invoice is already correct.

You would then repeat Steps 2 and 3 for each additional month, posting your duplicate Journal Entries in February, March, then April. Duplicating existing Sales and Purchase Orders and changing quantities will save time. Be sure to complete the Purchase for the month before the Sales for the month, to not run out of Inventory!

4. The final step is to correct the dates of the Sales Orders, as this is the only other report not yet correct. 

• Export the Order Reference and Order Date for all Sales Orders.
• Using import-compatible export.
• Edit the dates outside Odoo. 
• Import the file to update the dates.

After doing this (plan for several hours, including time to review your work) you will have:

Sales Report:

Purchase Report:

Balance Sheet:

Profit and Loss:

Note that your Inventory Report will be accurate only moving forward from May, you won't be able to run it for past months, so you would leverage your Balance Sheet to see your prior month end Inventory Balances.

Additional References for Accounting Go Live:

https://www.odoo.com/forum/help-1/how-to-go-live-with-accounting-complete-set-up-in-v15-197693

https://www.odoo.com/forum/help-1/setting-up-accounting-configuration-settings-is-there-a-checklist-of-things-i-need-to-manage-to-go-live-170049