Slice the cucumbers thinly - I always use the slicer on my @cusinartcanada food processor.  I don’t peel the cukes either.   Add the chopped onion and the salt to the cucumbers and let it sit - drain any excess liquid (very little from these babies). 

To make the brine, combine vinegar, water, sugar, garlic, celery seed, and mustard seed. Bring boil, while stirring occasionally. Gently boil for about 2 minutes until all of the sugar has dissolved. 

Fill your jars!   They should last about 8 weeks but in our house the go way quicker than that.