Step 1: Sanitize. If you have a spray bottle with starsan, then use that to spray the rim of the bucket/carboy. If you don't have starsan, I've read posts where people use vodka in their spray bottle.
Step 2: open your fermenter.
Step 3: Use a sanitized wine thief/ turkey baster(spray with your solution) to withdraw a sample of your yummy beer. Usually you need about 2 oz. to fill the tube the hydrometer came in(or a graduated cylinder if you have one). Close your fermenter.
Step 4: Add your hydrometer to the tube. There's usually foam on top, so set it somewhere to warm up and let the foam dissipate.
Step 5: After 10 minutes or so, give the hydrometer a spin so any bubbles on it clear off. Read the hydrometer at the meniscus( the bottom of the air/liquid interface).I've found a good light source behind the tube and reading glasses help!
Step 6: Have a sip of your beer. It won't taste exactly like it will eventually, but it gives you an idea. Don't pour it back into the fermenter.