Sooner or later you will find yourself saying something like "this would be so much easier/faster, if I only had a funnel." Well here ya go, the quickest, easiest and probably cheapest way to make a funnel for liquids. It's so easy, you could be done by now.
Step 1: Materials
You will need.
A plastic bottle I like 2 liters because they are wide bottles with narrow necks. but any old bottle will work.
A knife, razor blade is probably best but if it's sharp it'll work