The sound /b/ is a voiced bilabial consonant, similar to the sound at the beginning of "boy" or "buy".
The sound /æ/ is a short vowel sound, similar to the sound in "cat" or "man". It is important to note that this sound is different from the long vowel sound in "cake" or "name".
The sound /ɡ/ is a voiced velar consonant, similar to the sound at the end of "dog" or "big".
When pronouncing "bag", start with the /b/ sound by pressing your lips together and releasing a small amount of air. Then, move your jaw slightly downward and produce the /æ/ sound by opening your mouth slightly. Finally, move the back of your tongue close to the soft part of your palate to make the /ɡ/ sound.
Remember to practice these sounds individually before attempting to pronounce the word "bag" as a whole.