For most people, accent errors on TH result from incorrect placement and manner of articulation.
Your tongue should rest gently between your teeth, not behind them or pushing against them.
The air should flow and there should be a noisy friction sound. If you can't hold your th sound, you probably need to move your tongue and make sure that you are not blocking the air from flowing.
Use a mirror to help you see where you are placing your tongue as you try your TH sounds.
Practice on common words. Voiced TH /ð/:
Voiceless TH /θ/:
If words are too difficult at first., try syllables:
the thee thoo they thah though
eeth ooth ahth ehth ith oth
When you are comfortable using the American TH sound in words, move on to repeated phrase practice, for example:
those people, those shoes, those things, those words, etc.
with me, with him, with them, with the car, with the group, etc.