You will need more than one warehouse enabled in order to create a warehouse transfer.

There are two primary methods to create a transfer:

Start by clicking on Replenishment > warehouse location that will be the destination warehouse. This will show you the demand and replenishment recommendations specifically for that location. 

Select items that you wish to transfer. 

Create a new Transfer by clicking on the arrow to the right of where you would create a new purchase order. 

In your Transfer, chose the origin warehouse. The next field will have you chose a destination warehouse. By default the destination warehouse will match the warehouse used when originally viewing replenishment recommendations. 

Add products as you normally would on a purchase order.

If there is not enough stock available in the source warehouse, you will be warned at the bottom of the purchase order to Fix Insufficient Stock. This will update quantities ordered to show a maximum of the stock level in the source warehouse, within the limit of the replenishment recommendation (quantities ordered will only be lowered if you fix insufficient stock).

Once the stock arrives at the destination warehouse, you'll receive it just as you would a purchase order.

If you have enabled more than one warehouse, it can be helpful to see stock levels across locations. If you need to transfer stock, balance stock at various locations or compare inventory levels across warehouses, then the Compare Warehouse report can help.

First click Reports from the left menu, then Compare Warehouses.

Choose a warehouse at the top of the screen. 

This selection will correspond to information shown in Details. 

Then select which warehouses you would like to compare to this warehouse using the 'Compare To' button in the upper right corner.

Note: The date range of this report correlates to historical metrics such as sales and revenue, which you can add to your view using the gear icon in the upper right corner to select columns. The date range in the Replenishment screen determines how the forecast is calculated. The Replenishment date range will also determine replenishment recommendations and overstock.

Once you have added a second location using the 'Compare To' feature, you will see new columns for Available Stock and Total Available Stock.

Available Stock is inventory on hand at that location in excess of what will be needed during the days of stock (planning period). This metric is based on the lowest amount of stock needed (based on the forecast) during the days of stock. Any inventory available over that amount is identified as available stock. (Note that Overstock is excess inventory after the days of stock. Available stock is excess inventory during the days of stock.)

Total Available Stock is the sum of Available Stock from all compared locations. This field will appear as zero if no warehouse is set to compare to (since there is no stock available to transfer until you have a warehouse for reference). Once you set a warehouse to compare to, this field will populate.

Available stock can be transferred to another location to meet replenishment needs rather than order additional product from your supplier.

If you would like to see only variants that are needed at your warehouse (selected at the top center of the screen) and where quantities are available in other warehouses, use filters. First click Filters > Replenishment with a minimum of 1. 

Then add another filter for Total Available Stock to be at least 1. 

Finally, click Apply Filters to see results.

Once you've determined which products have available stock and can be used to meet replenishment needs at another location, you'll create a transfer. First select items to be transferred.

Then select '+ New Transfer for Variants'.

You will then select your Source warehouse (origin of available stock), your Destination warehouse (where stock is sent), and the Replenishment location/warehouse to use for adding order quantities.

Note: You can set a Combined Warehouse as a "Replenishment" warehouse to use replenishment recommendations generated using a Combined Warehouse. Since this is not a real warehouse, a true destination warehouse must be selected.

By default, you will limit the quantity of stock to transfer to the Available Stock in the Source warehouse. 

This helps to ensure that you aren't moving too much stock out of your Source warehouse.

Finally, click Create Transfer. 

At this point, you can adjust information and quantities just as you would with a purchase order or other warehouse transfer.

When you click 'Save' at the bottom of the page, the transfer is created.

On the Purchase Order dashboard, you can now see the transfer listed.