Swatching and blocking is an important part of knitting/crocheting wearables. It helps with adjusting sizes, straightening out the stitches, and giving your finished piece a more polished and clean look.
Blocking is extremely important when starting fitted pieces such as sweaters and other clothing (not so much with shawls) as a blocked swatch will give you more accurate measurements than an unblocked swatch.
There are a few different ways to block knits, immersion/wet blocking is probably the easiest and it's the method I use the most.
Step 1: BoM
Step 2: Soak 1
Fill a tub or container will cool water and add a drop of soap. Place your knit in the water, and let soak for 5-10 minutes.
Step 3: Soak 2
Remove your swatch from the soapy water and clean out your dish. Refill with cool water and soak the swatch for 30 minutes (or longer depending on the size of your knit). Remove and wrap in a towel to squeeze some of the excess water.
Step 4: Block
To block your knit to size, place your piece on a blocking mat and use pins to stretch the knit into the size desired. Don't pull or force your knit, you don't want it to look stretched.
Allow your knit to dry fully before removing the pins. You can always take a hair dryer to the yarn to speed things up.
Step 5: Unblocked Vs Blocked
The first picture is the same swatch before and after blocking. You can see the how much neater the swatch looks after blocking.