At high levels of play shielding is a very powerful tactic. In general, playing from a strong defensive position is generally tactically advantageous in comparison to being forced to fight on the offensive. Since shielding is such a vital part of defensive play, I'm going to quickly go over how to operate gameplay from shield.
When shielding, your options are limited. You can either roll, spot dodge, jump, grab, drop your shield, or continue shielding. Effectively, dropping your shield and attacking is also an option. However, there is a delay that occurs when dropping your shield. The following options circumvent the seven frame delay of shield drop animation:
Rolling is done by holding shield and tapping a horizontal direction on the control stick. It makes you temporarily intangible and moves your position. However, most rolls are slow and often can be punished on reaction.
Spot dodge is done by holding shield and tapping down on the control stick. It makes you temporarily intangible, but for a shorter time than roll without distance moved. It does have shorter cool down than rolls though.
Grab is done by holding shield and pressing the attack button. This is useful if a character hits your shield and goes through cool down directly in front of you.
Jump out of shield is versatile because it leads to multiple options. Immediately after jumping, once can cancel the jump's startup animation into an upsmash or an up special. Otherwise one can perform an aerial immediately after jumping.