I've never tried to install a shed on gravel. It's a carp surface for a shed base. All shed bases need to be level.
The felt needs to be pulled taught before it is nailed so that you don't have all those wrinkles that the water will blow under.
Your second picture has the nails in about the right place but you could do with another nail in each gap between the existing nails, so twice as many nails as you have fitted. If you put roofing felt adhesive between the two layers of felt where they overlap then you stand a chance of it being watertight (when the wrinkles have gone).