I don't have any first hand experience but you may like to check the first post of the FRA connection thread linked in my signature line.

When you arrive at a D gate, check the departure board to find where your flight to HAJ departs from and just follow the signs for that gate. As you noticed it will be either an A or a B gate. Following the signs should take you to the Skyline platform. If departing from a B gate get off at the first stop. If departing from an A gate get off at the second (last) stop. Don't just follow the letter of your departure gate but the letter and the number, since gates B1-19 are in a completely different area from gates B20-48.

If you follow the signs for your departure gate the route will take you through passport control and security. Don't look at a map of FRA to try and make sense since it is likely to confuse you more than enlighten you.