We would recommend these inexpensive stitching chisels from Seiwa for beginners thinking to start a leathercraft hobby.
Professionals also use these to make products due to their durability.
 
The iron creates diamond-shaped holes, the rhombus achieves neatly aligned diagonal seams. 
 
Available in sizes 2.5mm, 3mm, 4mm, 5mm and 6mm. 
Please note that Seiwa's stitching chisel sizes are measured between the points of the prongs.