My personal preference for filling a hip flask of any kind is with a funnel. There are flask-specific funnels available that look as classy as your best alcohol flask. However, I usually opt for collapsible silicone funnel options for any kitchen task. But any type of funnel will allow you to easily pour your favorite hard liquor. Some flasks feature a wide mouth opening, but a funnel still reduces spills. Some flasks may even feature an integrated silicone funnel, the epitome of easy pouring.