There's an easy way and there's a hard way. The easy way is to go to this web page:
Here's the hard way:
Starwood's website has a new glossy, fancy look to it that is very unhelpful, a classic example of design over function. I spent a good half hour this morning puzzling over how to transfer my SPG Starpoints to an airline.
If you are trying to do this, you might be led astray (probably purposely) by the SPG page that says this:
None of the choices on this page are what I wanted (or what most people will want.). Ignore this page and try not to get back to it. I was repeatedly directed to this page.
Instead, click on the Travel icon on the top of the above page. This takes you to this page:
This choice looks like what you want -- but it is NOT! This choice takes you to Starwood's own "redeem SPG points for airline flights" page which is not the "transfer my SPG points to an airline mileage plan" choice that most people want.
Instead, scroll down to the bottom of this page:
Finally, if you can get past and decipher all the clever, cutesie wording, then you will see that under "Sky's the limit" is what you want: Turn your Starpoints into miles.
Click on the weird down arrow thing to open up yet another page, and click "Learn More." Hopefully you will get to this page:
From this page, you finally get some direct language about how to transfer Starpoints to airline miles.