Do it indoors, if the roof and walls block GPS.
Under a sheet of metal or with aluminium foil covering the drone body.
You might need to also prevent your phone from getting GPS reception too.
Radio waves can't pass through water and trees are full of water.
Your control signal won't be affected by a branch or two but if you get enough foliage and timber between the controller and the drone, the signal won't get through.
What you are...