Leathers and finishes vary greatly. So, testing an area or similar materials before beginning is highly recommended. Bend and stretch the test mimicking normal wear. If cracking appears in testing, then follow the instructions for adding GAC 900 to acrylics for softness as recommended in the article linked below.
Remove any previously applied coatings. Clean area to be painted where needed.
Acrylic, oil, watercolors, and drawing media can be applied directly to the leather. However, the most important consideration is the end use. Acrylics are the recommended choice for wearables, but all applications should be tested.
Article: Painting on Leather with Acrylics