One of the more difficult aspects of sewing leather garments or just working with leather in general, is that the seams cannot be ironed flat as you would with regular fabrics in the dressmaking process. In dressmaking, ironing is perhaps the most essential tool is ensuring that all seams lay smooth and flat for a professional finish.
To smooth out leather seams, you can simply finger press and crease the seam allowance layers to smooth out the seam and encourage the seam allowances to lay flat. While this works well with faux leathers that have a crisp, structured finish and are conducive to creasing easily, most leathers (both faux and genuine) require a bit more "force":
Seam's can be flattened with a mallet designed specifically for use with leather and suede materials. A mallet is smaller and gentler then a conventional hammer which prevents unintentional dents and marks on the leather surface. Most leather-designated mallets have curved structures so you can smoothly drag them across the finished seam to further smooth and falten it.
While leathers cannot be conveniently ironed as you would with most conventional fabrics, one of the most satisfying aspects of working with leather is that the cut edges do not fray. In most cases, leather seam allowance edges are left unfinished and untreated. If you prefer to give the inside of the garment a more decorative finish, you can simply pink the edges with pinking shears as shown above.
Note: The seam smoothing techniques described above are designated for leather seam that cannot be topstitched. The most permanent way to keep leather seam allowances down and ensure a flat/smooth seam finish is to topstitch the seam allowance layers down- Topstitching should be used if your garment is deigned to be topstitched.