GPS starts here if you are using a phone to do it.
You need to constantly charge the phone if using it as a GPS on your handle bar.=======================================
You need a good cradle. Pick one that will NOT drop your phone -- I use a variant of this type of butterfly band cradle. I also use a safety string made up of heavy upholstery thread (strong nylon thread) sewn through the phone cover/case so if the phone gets loose it has the string and the USB cable to hold it to the bike until I can grab it.
Next, learn how to download maps from Google Maps. Having the map inside the phone's internal memory means you don't lose GPS functionality every time you get out of range of a cell tower.
Rig up some ear buds in your ears before putting on your helmet -- you do want to be able to hear the turns coming up, etc.
Lastly,
you will use up some data up doing this trick. To minimize this, start your guidance route while sitting in a burger joint using their wifi to get you all powered up, updated and running.
Don't cancel your route until you get there. This totally minimizes the amount of data you use, while keeping you all googlized on the map quality. Example: on the last trip I used 150 meg of data between calling my wife repeatedly and navigating here and there.
If you do lose your map due to messing about with clumsy fingers -- go find you some wifi, a bathroom and a senior drink at a Hardees or a Burger King and get it all back together again.