Start with a good tool that you only have to pull on, and don't have to grip in itself. This means correct sized spanner, quality adjustable spanner like Bahco, Footprint wrench or Stilson (if you don't mind teth damage on the nut)
Now you can properly feel for when the fitting is tight just so. fibre washers don't want to be nipped up in thge ifrst instance beyond the just so feel. Compression fittings need a good strong half turn after they grab, harder force required the larger the diameter of the fitting. Rubber you tighten up for England as it follows the shape of whatever you are sealing, and does so better the harder you compress it into that shape. It doesn't split.
In all instances it is essential to use blemish free pipe and olives/washers.
If these rules are followed the fittings usually 99% of the time are leak free when you fill up, but at worst there will be a slight weap, which on olives you nip up with another half turn, on fibre wahsers just tighten ever so slowly until drip just stops and no more or you will split the washer and have a flood now, or wound it and have a decorating bill in two weeks.
In the very last resort for an olive, if it still leaks, drain down, dissassemble, add paste. That'll fix it.