When we model a handle, it is preffered to have a curve on the one side, so that the fingers can hold it better... this is how we do that..
These are the steps....
first of all we take a handle that we need to model the grip...
then we go to the "right" plane (it depends on how you have modeled your part) and we create a spline to determine tha area that the grip will be...
Next we split line it to all faces...
then we delete the split line faces.... like this...
Next we draw a sketch like how we want the grip to be...
We create the surface using the loft surface feature (please note the constraints... tangent and normal to)
this is the result...
Knit surfaces (creating a solid body)
This is tha final result...