There is a huge difference in how different leathers take the indigo. Of course the leather should not be treated with anything. The key is to have the leather well wet before dyeing. A few hours in warm water to be safe. Brushing the leather with a soft brush in the water helps to get in wet evenly so the dye job will be even.
The indigo should be top condition. Keep the leather under the indigo for at least five minutes. Not the average one or two minutes. Oxidise for at least ten minutes then rinse the leather thoroughly and brush it lightly to take off any indigo pigment that is simple caught up in the fibres of the leather but not actually attached to the leather. Repeat this until you get the depth of colour you want. Remember the leather looks dark but will dry several shades lighter.
To neutralise the alkalinity of the indigo before it damages the leather and shrinks up nastily, wash and wash and rinse the leather. Use whatever kind of cream etc. that you usually use to keep some suppleness. If the leather is not taking the indigo well let the object dry and wait a few days before repeating the process.
The indigo makes a simple mechanical bond with the leather like it does with blue jeans. It will abrade off. This cannot be helped. Sealants etc. can help but the indigo leather will weather. Calculate this into your design aesthetic and make sure your client knows this well before selling.
Jonathan, (Bandana Almanac fame. http://bandanna-almanac.com) was here yesterday from Osaka to dye some leather boot uppers. They looked great and look forward to wearing a pair in a few months. The order will end up in Chicago.
Leather can successfully be dyed with indigo. It takes some experimenting and understanding the properties of both the indigo and the leather being dyed. It is possible to use stencil dyeing with leather. Don't expect crisp clear lines though.