It's very similar to the Spanish ñ--just move your tongue a little farther back in your mouth, and make the sound with the back of your tongue instead of the tip. In other words, first make a "g" sound. Now, keep your tongue in exactly the same place as it was for the "g", but breathe out through your nose instead of your mouth.

Spanish speakers already have this sound. They convert an "n" into the English "ng" sound in words with "n" followed by "k" or "g." Here is an example from Wikipedia. [ŋ] in cinco, venga = [ŋ] in sing. Send me a message if you want some suggestions for practice (I teach English pronunciation). Related note: English speakers similarly convert an "n" into the English "ng" sound in words with "n" followed by "k." For example: bank is pronounced "b-a-ng-k."

Thank you Gray and Liam for your advice

The only advice I can recommend is just to practice a lot. If possible, get a native speaker to help you by saying the words a few times (modeling the correct pronunciation for you). The native speaker can then listen as you repeat the same words, and then repeat them for you again, etc., until you start to get a feel for how to say the sounds. (I don't recommend doing what Liam has suggested. The sounds "sin" and "kin" are definitely not the same as "sɪŋ" and "kɪŋ.")