Oh. Well, people who created this skill decided to go with the word "ciasteczko" for a cookie (which for me seems strange as it is diminutive, I'd definitely go with "ciastko"), but if they did, then I guess we should keep the starred answers this way. Therefore, "ciastka" shouldn't probably be a starred answer, as it wasn't taught... it will just be an accepted answer now.
Well, the first sound is the most difficult. It is a palatalized c (English tz/ts), something not present in English. The rest simply goes: a-stech-ka. If you cannot pronounce this ć/ci sound, the closest will be ch: cha-stech-ka. Better this than nothing. And with time, you will learn to perceive and pronounce the palatalized sounds better.