In each half-litre (1 US pint) jar put: a few strips of onion, a piece of celery (2 to 3 cm / 1 inch), 1 teaspoon of mustard seed. Double for the litre / quart - sized jars.
Pack cucumber spears into jars, leaving 2 cm (½ inch) headspace.
Put vinegar into a large pot. Add salt, sugar, and bring to the boil on the stove (or put it in something large enough and microwave safe, and zap it till boiling. Be cautious of surging when moving microwave-heated liquids.)
Pour heated vinegar into jars, leaving 2 cm (½ inch) headspace.
If you are short of heated vinegar, just quickly zap some more.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process either size jar for 10 minutes; increase time as needed for your altitude (see table below.)