Thick leather is hard to cut...which is why it is so great for making things that need to withstand such things. If you are going to make one-pass cuts, it will take a lot of force. If that is the case, you have to either use a clamped down straight edge or some kind of jig that helps you control the blade. Remember that force and control are inverse properties here, and a slip could result in a really bad injury. The alternative is to make multi-pass cuts, possible with a smaller blade, and retain safe control of your tool. The downside to this is that the edges you produce may be a bit ragged. Tidying these up with a small sharp blade and/or a rounding tool is not hard. Always opt for control and safety whenever possible.