How do I make liquid glucose from powdered glucose
Well, the answer is "it depends." This is pretty much the same as asking, "I have sugar, and I want sugar syrup. How much water do I add?" It depends on the concentration you're looking for. If you're looking for a 24% solution, it's 24 grams of glucose in 76 grams of water. A 30% solution is 30 grams of glucose in 70 grams of water, etc, etc.

Unless you have some chemical reason to avoid dextrin, you can just substitute corn syrup. The only reason they use glucose in Europe is because they don't have our superabundance of corn.