Active dry yeast must be rehydrated in liquid before being added to the dough. The liquid could be water or milk with temperature of 43 - 46 C. It is not concentrated as instant yeast, so you need more active dry yeast when substituting.
Instant yeast, also known as fast-rising bread machine yeast is made into much finer particles. Instant yeast is usually more expensive and can be directly added to the dry ingredients without rehydrating or proofing first. You can use water that is somewhat hotter, 48 - 54 C, to make your dough unless the recipe calls for colder water.
Fresh yeast, also known as cake yeast or compressed yeast usually respond a bit quicker than active dry yeast. Fresh yeast is packed into small squares and it has a short shelf like of a few weeks, compared to months or even years for dried yeast.
Proof the fresh yeast by crumbling it into warm water (43C) with a tablespoon of flour and sugar. Stir and wait for 10 minutes until it foam. To use fresh yeast in a recipe, you can make a well in the centre of the dry ingredients, crumble the fresh yeast into the middle, cover with warm liquids and stir up a little of the dry ingredients into the liquids. Wait until bubles form.
You may find recipes using large amounts of fresh yeast which is not necessary. Simple bread recipes require 1% to 1.5% of the weight of flour. If you're using 1 kilo of flour, that means you need only 10 to 15 grams of fresh yeast.
Check out below a table to convert fresh yeast to active-dry yeast or instant (quick) yeast.