Standard recipe is one large egg yolk to 1 liter of water. It doesn't have to be that way, but that should be a ballpark starting point. You don't need starch, but if you use it, 10 parts flour to one part starch. Most pre-mixed flour has too much dry egg -if you use pre mix add flour to it. Water/egg mixture should be chilled. Whisking air into it so there are bubbles helps lighten the batter a lot - it will be easier when your bowl is a little dirty from previous batches of batter, but even with a clean bowl try to get some foam in there before you add the flour. Add flour and mix with chopsticks as little as possible in order to entirely coat the ingredients with batter - if you don't mix it up enough you will get patchy dry spots of flour when you pull it out of the batter, but you really don't need to mix that much. It should be lumpy, and dry spots of flour floating in the bowl are not a problem. If you stick your fingers in they should come out a little foamy with lumps and maybe some dry four - not like pancake batter. Oil is 160C for slower cooking vegetables and goes up to 180 for most seafood, including shrimp. Raising the temperature before you pull things out will help shed oil in the final product. Do not let the oil drop below 155 when you add your ingredients. Although many places do not dredge in flour before they drop something in batter, results are typically better if you very lightly dust ingredients with flour using a brush. Doing this can help hide some mistakes with your batter.
IMHO using carbonated water makes greasy tempura. Carbonated water gives the proper lightness to the batter, and it feels nice and light when you mix it, but the tiny gas bubbles that expand when the batter heats up trap a lot of oil that can't be shaken out. The resulting batter has a texture more like a Funyun than tempura. Some people swear by it, even some Japanese people, but I've never seen a tempura chef use carbonated water, and I believe that is the reason why. I tried quite a few times, and although I was able to make the batter feel right with much less work, the final product was never what I wanted.
Practice. A lot. I made tempura for a few months and the more I did it the more I realized how difficult it is. I now feel completely inadequate when I'm working with someone who knows what they're doing, because I realize all the flaws that experienced tempura chefs consider. Western chefs mostly think that it's just a matter of mixing ratios and temperature, but it is an art. The last time I ate tempura in Tokyo it was at Kondo, which is regarded as possibly the best tempura in Japan, and the complexity of what they are thinking was really impressive. The chef is able to make tiny changes to the batter for every different ingredient they serve. I saw the Kondo chef on TV one time and he was able to tell the TV hosts the exact temperature of the oil just by looking at it. When you stand in front of a pot of oil for eight hours a day for a few decades I guess you develop some superpowers. They broke some stereotypical rules though. They didn't chill the batter, and they didn't even shake the oil off after they pulled things out - but it was still completely dry on the paper. Of course their oil probably cost $20/liter (or more), and high quality tempura oil wicks off the final product very differently.
BTW I do not claim to be an expert.
