If you can get fresh yeast from a Baker or local shop (even a Supermarket), fine - use it.  If you can't, do this...

Makes 20g Fresh Yeast

Ingredients:

• 4g Sugar
• 10g warm water
• 10g Dried Yeast (I use the Allinsons, in a little yellow tin)

Method:

1. In a small bowl, whisk the water and sugar until disolved.
2. While still wisking, slowly add the dried yeast.
3. Wisk until a loose paste is achieved - you may need to add a little more water - but only a little!
4. Leave for 10 to 15 minutes, the yeast mix will have doubled in size.  Then use it.

Notes:

Yes, I know the measurements make a greater weight than 20g - it works though