Singapore Airlines (SQ) is a Star Alliance member based in Singapore (SIN) and is regularly voted the best airline in the world. Here is the thing you need to know before spending any time searching: Singapore keeps nearly all of its long-haul First and business class space for its own KrisFlyer program. Partners like United and Aeroplan mostly see economy, premium economy, and some regional business class.

Because Singapore Airlines is in Star Alliance, you can book its award space with partner miles, including:
The easiest place to search is the United Airlines website. United displays Star Alliance partner availability, including Singapore Airlines, and you can search without holding any miles.
Log in to your United Airlines account on united.com. If you don’t have an account yet, signing up is free.
Select the Book Travel box and fill in your trip details:
Then click Search.

Pick the flight option that fits your schedule. Watch the connection times carefully, as some options have very long layovers.
If you would like to search a different day, use the calendar to scan nearby dates. Regional business class within Asia shows up for partners far more often than the long-haul flights do.
Make sure you select the Saver Award, as standard awards cost far more miles and usually can’t be booked with partner programs anyway.

Choose your preferred flight and open its details. Write down the following information: flight number, dates, and cabin class.
Now book with the program your miles live in. If it’s United miles, book right there on united.com. For other programs (Aeroplan, LifeMiles, Miles&Smiles), check their website first, since most can book Star Alliance partners online now. If the flight doesn’t show, call the program and read the agent the exact flights you found. They should be able to see and book the same space for you.