Wash and slice cucumbers into desired shapes.
In a saucepan, combine vinegar, water, sugar, and salt.
Heat the mixture over medium heat until dissolved.
Add crushed garlic, dill seeds, mustard seeds, and pepper flakes.
Bring the mixture to a boil, then remove from heat.
Place sliced cucumbers and onions into a clean jar.
Pour the hot brine over the cucumbers and onions.
Let the jar cool to room temperature for about 30 minutes.
Seal the jar and refrigerate for at least 24 hours.
Enjoy your homemade refrigerator pickles within 2-4 weeks!