You will want to use a high DE pectin. The pectin will set by solids, temperature, and pH. Typical solids is 78-80%, this will dehydrate the pectin enough for it to set. The temperature needs to be kept hot until deposited or sheeted, otherwise you will get pre-gelling. The acid, lowering the pH will set the pectin, pH should be 3.1-3.5. As soon as the acid is added, deposit or sheet as it will start to pre-gel quickly. Lower solids and higher pH will give a slower set, high solids and low pH will give a fast set. There is also a lot you can do with the pectin type, not only deciding on apple or citrus, but the DE (also called DM) as well.
Citrus pectin is typically a cleaner, clearer gel.
You also want to look at how much protopectin you're adding with the fruit, if any, and compensate with the level of pectin you're adding.
One final note, be sure the pectin is fully hydrated before you cook.