Follow these steps to configure a transit gateway to communicate between two VPCs that do not have a direct peering relationship:

1. Log in to your account and go to the VPC management console.
2. Scroll down to the bottom of the menu on the left-hand side of the screen and click Transit Gateways:

3. Click Create Transit Gateway.

4. Fill out the Name tag and Description for the transit gateway:

5. In the Configure the Transit Gateway section, leave the defaults as they are:

6. Click Create Transit Gateway.
7. Immediately after creation, the gateway will be in a pending state:

8. Once the gateway is available, select Transit Gateway Attachments from the left-hand menu and click Create Transit Gateway Attachment:

9. On the following screen, select the transit gateway from the Transit Gateway ID dropdown. 
10. Give the attachment an Attachment name tag.
11. Select one of your VPCs in the VPC ID dropdown.
12. Select the subnets from that VPC.

13. Click Create Attachment and then repeat steps 9-12 for the other VPC.
14. Go to Route Tables under the Virtual Private Cloud menu.
15. Choose one of the VPC route tables and add a route for the IP addresses in the other VPC to point to the transit gateway target. Do the same for the other VPC.
16. Test the connectivity between the EC2 instances you created in each VPC. Note that the ping command will be blocked by NACL firewall rules that prevent ICMP traffic.
17. Once you've finished, delete the Transit Gateway to avoid any future charges.