I would add it into the skimmer--slowly--breaking up clumps as you go. Don't want to put a big chunk in all at one time and chance a clog in the pipes, but by putting it into the skimmer it will go into the filter and dissolve more fully. Should reduce the possibility of undissolved powder causing cloudy water problems that may happen by putting it in front of the return jet. What kind of filter do you have? You might want to give it a good cleaning before adding all the Borax to it, to make sure you don't have to backwash any of it out.
Janet
Edit: One other suggestion: you might want to take a 5 gallon bucket of your pool water and add a measured amount of Borax to it, to see how much it takes to get that sample up to above 7.0. That might give you an idea of how much Borax you're gonna need.....
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