The LS-factor is an important variable to estimate average erosion - annual soil loss (as in RUSLE), and based on experimental data. It becomes greater when the slope is steeper and the effective slope length is shorter.
Slope length is the horizontal distance from the ridge to the point where deposition starts to occur. At a given cell, the length to the nearby steepest cell is successively calculated unless all nearby slope is less than half of their own slope.
A suggested overall workflow with a pair of input/output and corresponding SAGA tool are summarized below. As you see, there are many steps and involve several intermediate products.
Slope, Aspect, Curvature tool
Output: Slope (measured in radian)
Fill Sink (Wang & Liu) tool
Output: DEM without sink
Catchment Area tool
Input: DEM without sink
Output: Flow Accumulation (Total catchment area)
Flow width and specific catchment area tool
Input: DEM without sink, Flow Accumulation (Total catchment area)
Output: Specific catchment area
Input: Slope, Specific catchment area
To speed up the process, you can try
Basic terrain analysis tool, which is in the SAGA - Terrain Analysis - Morphometry group. It does all the necessary steps automatically to produce LS-factor.