Decide what you need grenadine to do: deep red color, tart fruit tang, or mainly sweetness (for baking).
Start with the ratio for your chosen substitute (use 1:1 when switching to a syrup/concentrate; use ¾:1 for very intense options).
For drinks, mix thoroughly and taste after 1 minute; for baking, fold/blend in, then check the batter/glaze once mixed.
Adjust: add a small splash of lemon juice/vinegar for more tang, or add 1–2 teaspoons sugar per cup of mixture for more sweetness.
If the substitute is gritty or chunky (jam), strain it before using in cocktails for a clean look and finish.