First, go to the Sounds of Speech website:
http://www.uiowa.edu/~acadtech/phonetics/english
Find the front vowels, and study the /ae/ sound, noting what the jaw movement and back-of-tongue movement. This sound, /ae/, always occurs in stressed syllables.
Then open
http://m-w.com
Type the word “ladder” into the search box. Find the definition and the speaker symbol, and listen to the recorded pronunciation of [ladder].
The first syllable is the stressed syllable: it must have a longer duration, a higher pitch, and more energy than the second syllable. The vowel in the first syllable must be clear and strong and long, thus we say a clear /lae/, which is a very articulated vowel requiring jaw drop, lip tension, back-of-tongue movement. The second syllable [der] is not as important, so this vowel is shorter, softer and relaxed. It is actually a schwa+r. At the end of this word, we want our tongue to be relaxed, and resting in the bottom middle of the mouth, so we are ready for to make the sounds of the next word.
Lower your jaw while keeping your tongue very relaxed at the bottom of your mouth, laying up against your lower teeth. Your tongue just follows along downward because your jaw is going down. Then, to make the sound, push down the back on your tongue just enough to make the sound. The sound is created by voicing your vocal chords. Your tongue pushes down to contour the sound. Your jaw drops down to let the sound slide out of your mouth.
If your /ae/ sounds too nasal (you can record yourself and listen or ask a friend), try putting a finger softly alongside your nose. Play around saying /ae/, to find how to make the sound with the least nasal vibration you can possibly feel with your finger. This vowel should come from your vocal chords and out your mouth, not up through your nose. We vibrate /ae/ in our throat (voicing) and then just allow the sound to exit our mouth through our lowered jaw and open lips.
Peggy Tharpe teaches, coaches, and publishes about English pronunciation and intonation. She believes that if you understand why something is happening, you're better able to address it and change it. She teaches the "why" of pronunciation as well as the "what" and "how".