I created 2 VPC's in the same region ( each vpc has - 2 public , 2 private subnets , 1 public and 1 private route table ) \ After that click on transit gateway
Creating 1st Transit Gateway
Enter Transit Gateway name and Description
UnCheck Default Route table association and Default route table propagationThis gives more control on routing
A CIDR for the transit gateway must be provided . This must be unique to VPC CIDR of all the transit gateway attachment VPC's
Since in this case VPC CIDR start with 10 and 172 . I choose CIDR with 192
Click "Create transit gateway"
Click "Create transit gateway" to create 2nd Transit Gateway
Enter Transit Gateway name and Description
Disable route table association and propagation
Provide the CIDR for new transit gateway . Keep this unique to that of other transit gateway
Click "Create transit gateway"
Create Transit Gateway Attachments with each of the VPC's . Project1 VPC is attached to Transit Gateway 1
Click "Create transit gateway attachment"
Click "tgw-0c6c9842be062ee49 (transit-gateway-region1)" ( choose the transit Gateway and type of attachment since we are attaching to VPC it's of type VPC
Select the VPC . for transit gateway 1 the vpc is project1-vpc . When selected we are prompted to selected subnets . Select atleast one subnet from each AZ . This enables transit gateway to reach the resources in that AZ
Click "Create transit gateway attachment"
Create Transit Gateway Attachments with each of the VPC's . Project2 VPC is attached to Transit Gateway 2