I've found three general takes.

1. In sheet music, the most common is just to write "W___", where the bar extends under the relevant notes/rhythm.

(Image source)

2. In lyrics-only websites, it is also most common to just use the letter "W".

(Image source)

3. Less frequently used is "double-u", but there are examples.

(Image source)

4. I found no examples of "double-you", and would personally find that confusing at first glance.

I've found three general takes.

1. In sheet music, the most common is just to write "W___", where the bar extends under the relevant notes/rhythm.

   
   (Image source)

2. In lyrics-only websites, it is also most common to just use the letter "W".

   
   (Image source)

3. Less frequently used is "double-u", but there are examples.

   
   (Image source)

4. I found no examples of "double-you", and would personally find that confusing at first glance.