Valid for GIS 3.2 (Bonn).
1. Do not use grass extension anymore, LAStools are the right way. Download LAStools and unpack them in C:/LAStools, or use plugin repository to get it. In any case I use version 1.1. and 1.2.
2. In any case you should have LAStools in your QGIS toolbox
3. Use file – conversion tools to get .las/.laz output.
4. Use file – raster derivatives to generate terrain rasers.
Few links worth to mention: