Measure 63 grams of your sodium chlorite powder or flakes and add that to 1000 ml of distilled water. Stir to mix and completely dissolve. You now have 1 liter of 5% sodium chlorite, or at least it will be close enough for your purposes.
If you want to be totally accurate you would use 62.5 grams of the sodium chlorite power or flakes and add that to 937.5 ml of distilled water.
Sodium chlorite power or flakes are 80% pure. So the mix will be 6.25% by weight but factoring in the 80% purity it comes out to 5%.