The flap T is similar to the raa' in Arabic, with the difference being that the raa' is rolled/trilled and the T is not. However, they're both alveolar sounds, produced by placing the tongue behind the top teeth. You will sometimes see people describe the flap T as similar to the D, and that's because the D is produced in the same place, although it's an alveolar stop and not a flap or trill. (Although the D becomes a flap, like the T, when it's between vowels, which means the D and T have the same sound when they're between vowels.) Basically, think of the flap T as somewhere between the D and the Arabic raa'.

Read/watch this: http://rachelsenglish.com/flap-t-really-d-sound/

It'd be easier if you could upload a video or an audio saying some words including the flap T, to spot what are you doing right or wrong.