Gather the ingredients.
In a mixing glass, muddle the watermelon to extract its juice.
Strain out the fruit pieces and any seeds, leaving only the juice.
Pour the vanilla syrup into a frosty pint glass, then fill halfway with watermelon juice.
Top with the beer.
Serve and enjoy!

Tips
When straining the watermelon, you can get a little extra juice by pressing the fruit into the strainer. Be sure to use a fine-mesh strainer to capture all the seeds and small pieces of fruit.
If you have a juicer, use that for the watermelon to maximize the juice yield. Store any excess in a tightly sealed bottle in the refrigerator and use it within a couple of days.
You can find vanilla syrup at most grocery stores because it's a popular flavor for sweetening coffee.
Vanilla syrup is also very easy to make at home. All you need is 2 cups of sugar, 1 cup of water, and a vanilla bean. Stir the sugar into boiling water, reduce the heat, cover, and simmer for 15 minutes. Pour the syrup over a split vanilla bean in a heatproof jar. Let it steep for about 8 hours before removing the bean and bottling the syrup. It's even easier (and cheaper) with 1 teaspoon of vanilla extract added to the hot syrup.
Watermelon beer can be made in any tall glass, no matter the volume. The point is to fill the glass half-way with the juice and syrup mix then fill it the rest of the way with beer.
You can certainly add ice if you like. It's not necessary if you use a chilled glass and ice-cold beer, though.