What are you using to sanitize? If you are using a no-rinse like one-step, iodophor or starsan, they're called "no-rinse" for a reason. These sanitizers, for one thing are "wet contact" sanitizers...once the surface is sanitized, as long as the object is wet with sanitizer, it is protected from any particulate matter landing on it...rinse that away and you've effectively killed half it's usefulness.
In my airlock I use sanitizer, but vodka is often used...some people use tap water and don't worry, it's a matter of preference.
To measure by the gallon, without using a gallon? Have any kitchen measuring cups? I have a pyrex quart measureing cup that I use, knowing that 4 of them equal a gallon...
you can use anything...pitcher, bucket...just sanitize it first if it's for top off water...If you have a container but don't know the size, and have a measuring cup of any size, (like a 1/2 cup) then you can keep filling the container until you can get an amount you know, and mark it with a sharpie... If you know one volume of water, then you know how many times you'll need to fill it to achieve 5 gallons.
Of course if you've marked your fermenter with gallon increments, then it doesn't really matter, you go by that.
Just remember to relax, and breath...this ain't brain surgery your doing, it's making beer, and it's really hard to ruin your beer.