Archaeological finds from the period include simple embossing and surface cutting techniques.
A lot of re-enactors use leather carving techniques similar to modern methods but I don't know of any real evidence for this in Viking / Saxon finds.
There is a good knife sheath that was found at Coppergate in York where conditions were really good for leather finds. There is also a similar find from Gloucester.
The most common form of decoration found at York is impression with a simple tool. Ring and dot punching is also known and some of the more elaborate sheaths show embossing work.
The best general source of information I have found is "Leather and leatherworking in Anglo-Scandinavian and Medieval York" by Quita Mould, Ian Carlisle and Ester Cameron.