Where should I put my phone while it's recording?
At Slopes we tend to keep our phones in an inner jacket pocket where it sits on the chest. If it's really cold out there don't stick it in an outside pocket, you don't want your phone to freeze and a cold phone can not only reduce GPS accuracy, but battery life too.
Just don't put it somewhere in your backpack. For GPS to work it needs to be somewhere where it isn't too obstructed or the signals won't reach it.