Either a funnel, which you can find for about a dollar at walmart, or I usually just measure it out and carefully pour it from the measuring cup into the bottle.
The threaded type Al's illustrated work well and seal the threads in the event you might overfill the bottle. These work great as most plastic bottle threads are 25mm or 26mm. I also have a couple of plastic funnels I use for bottles without threads or with threads of a different size...
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