Simple Glass-Cleaner recipe:

• 1/4 cup white vinegar (apple cider vinegar will work as well)
• 1/4 cup isopropyl (rubbing) alcohol
• 1 Tbsp cornstarch (reduces streaking)
• 2 cups water
• 8-10 drops essential oil of choice (optional)

Method:

1. Combine everything in a spray bottle.
2. Shake well to mix.
3. Spray onto glass surface and wipe clean.

TIPS & NOTES:

• Use old newspaper to clean your windows with any cleaner. You can also use microfiber cloths or rags, but try to avoid using paper towels which are not only terrible for the environment, but heavily processed and bleached.
  
• Shake well before using since the cornstarch might clog up your spray nozzle otherwise. Allergic to corn? Other starches like tapioca or arrowroot will work as well.
• Make sure to label your bottle. Some home made cleaning products don't react well with certain surfaces (for example, vinegar and marble surfaces do not mix!)

You can also use a basic solution of vinegar and water to clean windows, but I find that it tends to leave streaks that can be hard to remove.