Weather-wise, the best time for a Dubai hop-on hop-off tour is between November and March. This is also peak season, so expect some crowds. The heat is scorching from April to October so avoid those months.
Depending on your travel plans, you can choose from 24 hours, 48 hours, or 72 hours.
You can board the bus at any of the stops along the route. Just show your smartphone voucher to the driver. Major stops can be found at The Dubai Mall, Dubai Museum, and Heritage Village, among others. Please keep in mind that some stops are new and have yet to be clearly demarcated. Don't be shy. Feel free to ask around if you aren't sure that you are in the right place.