In addition to your garment, blocking wires, and stainless steel pins (if the pins are not stainless, they will rust and stain your garment), you will need a flat surface large enough where your knitting can be pinned out and left to dry. Hardwood floors don't work at all, but carpeting (when covered with clean, dry towels or sheets) and spare beds work just fine. Some people also use blocking boards. I have carpeting, and often a spare bed, so I use those.
First, get your knitting good and wet. This means let it soak completely submerged in water for at least 15 minutes—30 minutes is better—the blocking will go much better if the yarn's core is thoroughly wet. Drain it, gently squeeze the first batch of water from your garment (do not twist or wring), then wrap it carefully in a towel to soak up most of the excess water.
Essentially, all you do is spread the knitting out, run wires through the straight edges, pin it down, and leave it until it is completely dry.
When blocking sweaters wires go along button bands, hems, and sleeve edges, and any other edge that needs to remain straight. Use a yardstick to ensure sweater pieces are blocked to the expected dimensions, matching pieces are the same length (both sleeves match, sweater fronts and back are equal from shoulder to hem), and that the pieces are lying straight and flat.
When blocking lace shawls and scarves, wires go along any straight line—top and/or edges—and along points that need to be pulled out evenly. When blocking lace you will also need to stretch it in order to get the most definition from your stitches. I usually pin it out, stretch it, move the pins, stretch it some more—again using a yardstick to ensure the sides are of equal length, edges the same width, etc.—until the shawl is slightly larger than the desired finished dimensions. The lace will shrink a little bit after it has been unpinned, and has had a chance to relax.