As you've already discovered, leather is difficult to cut as it burns rather than cuts cleanly. Engraving tanned leather usually has good results but can also smell pretty bad. It will probably work better if you have the focus tuned.
To determine the focal length of your lens (likely 2.5" or 3") via trial and error you can use paper or Kapton tape and test different heights and determine the smallest spot size, then fine tune until you get the optimum result. A microscope or magnifying lens would help (100-150um is typical), the goal would be to minimize the heat affected zone (HAZ) and hole size, which indicates you've found the true focal length of the lens.
Alignment is critical to achieve the rated power output, placing a piece of masking tape carefully over the hole into the final mirror/lens housing and using a low power 'pulse' to create a small burn mark will indicate if you have the mirrors aligned - this needs to be done with the shortest and longest laser path. If you need to make adjustments (which can be tricky the first time) you would place a piece of cardboard or wood over the mirror and align each individually - several of the vendors have videos and instructions on this procedure.