Crafting Cheap Recipes: Check if they require an Orange salvage, if so, don't bother crafting them. There are some that use just white and yellow salvage. Now remember the purchaser doesn't check what was in the recipe so they'll usually pay >750k for something that cost considerably less to craft.
Make a note of what worked for you and repeat it over and over.
Crafting Expensive Recipes: When you have ~50m I'd have a go at doing purples. Just remember that the orange salvage and crafting cost (it's on the recipe, it's about 500,000 inf) mean it'll cost at least 1m to just craft a purple! So add that to the cost of the recipe, compare that with how much they're going for. The price of recipes varies over time and a lot of old sellers are stuck with loads of enhancements they crafted and stuck on the AH when recipes were more expensive, so be aware of that if the price rises you will 1. get good profits but 2. have sudden resistance levels where all these old players' sell orders suddenly become relevant.
My advice is craft recipes into enhancements and sell for at the VERY least 500,000.
For slotting my own characters, I always buy Attuned Enhancements so I can go up and down levels. I don't understand the enhancement system, it's far too complex for my little brain. I keep it simple.