First of all in a large vessel, add water to sugar and keep it on medium flame in order to make sugar syrup.
Meanwhile, take all the dry ingredients – fennel seeds, black pepper corns, cardamom seeds, and saffron, and grind them to powder in a grinder.
To this powder now add soaked almonds, pistachios, cashew nuts, melon seeds, and poppy seeds. Add about a cup of sugar syrup (or as required to help in grinding) and grind everything to a smooth paste.
When the sugar has dissolved and syrup starts simmering, add this paste to the syrup. Allow it to simmer for few minutes.
Remove from gas. Pass this solutipn through a seive to get smooth syrup. Add rose water and a pinch of saffron strand (optional). Mix well and allow the syrup to cool down. Once cold, fill it in clean glass bottle. Close the lid tightly. This syrup remains good for about four months and maybe more, in refrigerator.
Pour a small amount of syrup in a glass (less than a quarter of the glass). Add chilled milk to it. Stir it. Decorate with pistachio slivers, saffron strands, and rose petals (optional). Serve chilled.
Use thick milk to make thandai. Serve chilled. If the milk is not cold enough, add some ice cubes, though it dilutes thandai a bit.
You can replace milk with almond milk or soy milk.