To activate symmetry turn on axis checkbox, e. g. From now transform tools (Move, Rotate, Scale, Brush), that use Edit SOP will be adjusted accordingly, and the points on either side will be deformed symmetrically.
After that, you need to select one of two Launcher tools: Symmetrize and Clip. This will add the Symmetry node that will fix geometry issues.
Center seam distortions can be fixed by dragging Parm Slider. This will change the value of the Fix Seam parameter.
If you want to change symmetry center, select the Origin tool. This operation will ask you to select a set of points, after which the center of symmetry will be changed.
Another way to work with symmetry is to model one half and automatically display the mirrored copy in a separate object node. To display the Instance node as subdivision surface, select the SubDInst tool.
To deactivate symmetry, disable the axis checkbox.