The OpenSpf organization used to provide a tool for creating an SPF (Sender Policy Framework) record for mail server/domain. But that tool has been retired.
Microsoft offers their own Sender Id/SPF Wizard tool that is very easy to use, as seen in these screenshots:
Step 1: Enter your domain
Step 2: Obtain current publicly published DNS records
Step 3: Configure your SPF options
Step 4: Generate your SPF record
Now go to your domain control panel or contact your vendor to add your SPF record. You may then test your SPF by sending test emails, as described on the OpenSpf organization’s website.