Hat tip to LimeStuff.co.uk for these great tips:
1. Start with an empty mixer.
2. Add one part sand
3. Followed by one part lime
4. Followed by two parts sand.
5. Mix dry for at least five minutes.
6. After five minutes slowly add water until the desired consistency is reached, it is very important not to drown the mix by adding too much water.
7. Once the desired consistency is reached mix for a further twenty minutes. (The above example is based on a mix ratio of 1:3). The mix, to begin with, will appear dry but as mixing time increases the mortar will become much ‘fattier’. If too much water is added the risk of shrinkage will increase and the final strength reduced. Do not use any plasticisers.
http://www.limestuff.co.uk/pages/user-guides/how-to-mix-hydraulic-lime.html