Whilst it is possible to occasionally find acorn cups on the forest floor to use for a whistle at any time of the year, outside of autumn they can often be a bit manky. If you can source one great, if not there is an everyday substitute.
The short stem of the leaf and the long stem of the acorn tell us that this is a Pendunculate Oak (think of a pendulum). It's latin name is Quercus robur. The stalk arrangements of Sessile Oaks (Quercus petraea) are the other way round.
Pull the acorn from the cup. I aim for a fairly deep one as I find them easier to hold as a whistle, small ones are a bit bothersome I find.
The 'Urban acorn cup', or one litre fresh juice carton lid. Actually pretty much any plastic lid will do as long as you can get a seal on it, but more later. Metal lids tend to be a bit sharp around the the rim for use as a whistle.
Place the cup/ lid between index finger and thumb as shown.
Place the other index finger and thumb (as shown) and postition your digits until you have left a small inverted triangular gap at the top between your thumbs whilst making sure that the rest of the cup/ lid is sealed and airtight. You may have a bit of trouble with the last detail of the lid is too big.
Place this arrangement to your pursed lips so that your thumb knuckles are to your mouth (as shown) but with a slight gap at the top so that you can blow air towards the inverted triangular hole. You may find that you need to re-adjust your thumbs but it is quite shrill when you get it right.