Each of us have specific shields we use for specific things.
part of being able to consciously shield ourselves is being able to see through our minds eye what we require to happen, happening.
this requires little or no practice as all it takes is a little visualisation and a lot of faith in what we are doing.
some of us may feel we have a lack in the imagination department but all you really need is to see what you want and believe its there so no amount of a lack of imagination really counts.
im not going to post any of my personal shields as they are as said personal
but a simple one is as follows
close your eyes
which ever element you work best with see that element in its raw form (e.g. water rippling like the surface of a pool) around you in a complete sphere.
(continuing with the e.g. element)
feel the cold and the essence of the water around you
solidify the water as ice
believe what your seeing and feeling to be there and impenetrable
with practice you can alter or even add layers to said shield
peace to all