What do I do with this spice mix?

"Mix with...required salt" is just a kind of awkward way of saying to mix in salt to taste. Might make more sense if you read it as "the salt required to make it taste how you want".

Curd is basically a thick yogurt.

As for how to use it... I certainly might try their recipe and see if you like it (whether it's canonical chicken tikka masala or not).

But if you want to a variation of your existing recipe, just omit all the spices from that recipe and use this spice mix instead, probably starting with the same quantity as the total ground spices from the original recipe. Since the spice mix already contains salt, I might also omit the salt from your recipe and then add salt to taste once you've added enough spice.

I would suggest that you make a base gravy using onions and tomatoes to enhance the taste of the dish. For that you have to heat some oil in a pan and bring it just below the smoking point, add onions, ginger and garlic, bay leaves & dried red chilli(easily available in retail stores) to the oil and fry until the onions become translucent and garlic loses its raw aroma. Now add red chilli powder(small amount), turmeric powder ,salt(to-taste) and the Meat Masala. Mix them and fry the ingredients on medium heat until you see the oil coming to the top of the gravy. Once that happens, add a little water and put the marinated meat pieces(as explained above) to the gravy and cook until meat becomes tender. For finishing sprinkle some cilantro. There you have your tikka masala recipe v2.0 .