As is the case in most of the traditional Latin American dances, you should learn history and language of Merengue dance first to better understand it. The dance is based on a variety of steps, and combo chase or combo sweetheart requires a lot of understanding and practice.
If you do not know at all about the Merengue dance before hitting the dance floor for the first time, it is better to start with a learned partner. They can tell you in detail how you should be taking the combo step to chase in the dance. The technique involves actual release and then getting of the partner into sweetheart.
Also, if you start with some basic steps first and then gradually learn and grow towards combo chase steps, it will help you to improve your rhythm as well. The rhythm and coordination in all the steps in Latin American traditional dances are their basis as well as their charm. No rhythm no dance, and you can find yourself all over without going nowhere at the end. Better start with basic steps with your partner.
You can watch videos online or DVDs of the dance and learn about the combo steps. However, you will need a lot of practice if you want to learn quickly. Watching dance content can help you improve your imagination, observation and confidence, but actual practice should help you know if you can dance on those steps at all.
The combo steps involves a guy dancing with a girl, releases the girl backward while himself steps forward, with little steps. He has to then wait flip-flop for the girl to turn hammerlock, and the guy with a swing of the hand gets the girl into sweetheart. This appears simple but requires a lot of concentration and effort, and if you do not have any practice at all, it is difficult to perform.