So, here goes:
Ground and Center
Choose a quiet, undisturbed location. Outside is best, but this will work indoors, as well.
Sit in a comfortable position on the floor (though a chair may be used if you cannot get down/up). Place hands flat down on either side; if using a chair, place hands flat on thighs, and feet planted on the floor. Close your eyes.
Focus on feeling a connection to Mother Earth. Visualize the flow of energy up from the Earth, through our body and down through your arms, hands and fingers abck to the Earth, in a continuous cycle.
Continue the exercise until you are feeling calm and focused, and refreshed; all excess energy safely grounded and returned to the Earth.
Slowly break connection. Stop the flow of energy. Lift hands, take a deep breath in, open your eyes, breathe out. Stand up.
Visualize a wall, made of any impenetrable material (plastic, glass, brick, energy, light, etc.) that negativity, or any other harmful thing cannot pass. Feel that this wall is surrounding you. Check it regularly to detect any weak spots that may need reinforcement. The more you use your shield, the less energy and attention it will require to maintain itself. Over time, you may find that the "appearance" of your shield changes. For example, if you started with bricks, you may see that it is now pure energy, or a web of energy. It may have been clear, but now has blue tones to it. This is fine, as long as it works for you. If it stops working, tear it down, and build a new shield.
Well, these are taken from my own BOSN (Book of Shadows Notes), and have worked for me for many years.
Of course, once you're comfortable with these, or any methods, you should be able to do it "on-the-spot" as needed.