Use the Ctrl or Cmd key.  While sketching, Fusion 360 tends to try to read your mind.  Sometimes it does a good job but other times it completely misinterprets your your intentions.  To prevent Fusion from setting automatic constraints, simply hold down the Ctrl key or Cmd on a Mac and no constraints will be added to your sketch.

When creating a sketch on an existing body; instead of selecting the surface of the body directly, first create an offset plane on the surface and give it a distance of 0.  Then create your sketch on the offset plane. However, remember to untoggle the visibility of the bodies plus any sketches directly behind your offset plane. Otherwise you can still unintentionally project geometries and set constraints

Go to your Preferences settings by clicking on your name in the upper right hand corner of the screen and select Preferences.  On the left column, select Design and then uncheck “Auto Project Geometry on Active Sketch Plane”. Click Apply and Ok. Now, when you create a sketch on an existing surface, Fusion 360 will not automatically project the geometry over to the new sketch.  Going with this method means you won’t have to worry about tip # 1 and # 2 to avoid setting unintentional constraints.

I hope you found this helpful.  See below for a short video of all the tips mentioned.  Enjoy and visit desktopmakes.com more Fusion 360 content.