To bring the markers to your cursor, click in the timeline above the Qtractor workspace. Both the In- and Out- markers will appear at your cursor's position.

The position of the markers is the out-point for what will become your selection. Move your cursor left and click again to set the in-point.

The space between the two blue markers is now the active range. Click the Edit menu and use the options in the Select submenu to select areas based on the currently active track, the range spanning all tracks, and so on. You can also use this range to define a loopable area, or to delete or copy and paste its contents.