How to prepare a solution of accurate concentration?
This is how I did it in my analytical practicals, works for primary standards with a decent accuracy, works for everything else with a larger error. (You'd want to check concentrations with a primary standard then.)


Determine the mass of the empty volumetric flask
Prepare approximate mass of the substance you want the solution of in a beaker or watch glass or similar
Transfer the prepared substance into the volumetric flask
Determine the mass of the filled flask; the difference will give you the mass of the substance
Fill the flask with the solvent.
Given the precision of the instruments used, you can calculate the concentration quite accurately. If necessary check the concentration with another method.