I get asked all the time "Do I really have to block this?" And, then, followed by "How do you block something?"  I don't know why this simple process is so cloaked in mystery, it's no big deal and makes a huge difference to the look and drape of your knitted pieces. 

Here's what I do:
Fill the sink with cool water and add some Soak.  Soak is a biodegradable, phosphate-free and eco-friendly wash available in an assortment of delicate fragrances plus Scentless.  It's our favorite wash, the scented ones smell great, and it's the one we sell at Knit1. 

Dunk your item and swish it around.  Don't manhandle it, just get it thoroughly wet-- you can more easily damage the fiber while it's wet.  Once it's saturated, drain the water in the sink and press your garment to the sides of the sink to squeeze out some of the water.  Don't twist or wring the garment, just press and flatten it as much as you can.  Next, lay a thick towel out on the floor. 

I get the dogs out of the room and spread the towel on the bathmat for extra absorption.  

Take your garment out of the sink, making sure to support it-- you don't want it to sag and pull on the stitches-- and lay it flat on your towel and roll it up jelly-roll style.  

 I use all my weight to press as much water out of it as I can.  If it's really soaked, you may have to do this again with a second towel.  

Next, lay the garment on a flat surface covered with yet another, dry towel.  I have a blocking board, which is a great invention.  It's essentially a padded board with a slightly absorbent cover, kind of like an ironing board, but I have used everything from the bed, the dining table, the kitchen counter to the carpeted floor.  
Jessica's Adaline

As you lay your piece out, make sure that the edges are all nice and straight, and your stitches are laying flat and even.  
Noro Scarf

It's at this point that you gently manipulate the garment to your required measurements.  You'd be surprised how many extra inches you can get while something's wet!  Once you have it nicely laid out to the dimensions you need, walk away!  That's the hard part.  It takes a couple of days to dry.   There are special cases like lace, openwork or cables that may need some special treatment, but this is the basic process.  If I make a garment in pieces rather than in the round, I will block all the pieces before seaming.  It makes the finishing so much easier.  Just these simple steps will help you make all your knits look professional.