There is a error in the first part where you have used the words these and piece. These is used when referring to one item - yet piece is singular (just one item). if you made just one item it should be this unique piece, if more than one item it should be these unique pieces. With regard to your question about DID and DON'T Yes you're right they are two different tenses. Whether you use the same tense for both parts of the sentence depends on the meaning you are intending to get across. Ideally I would say it should be " I used a tree branch from the forest but I didn't harm the trees. I only picked up sticks which had fallen from the trees" By using the same tense you would be meaning that you didn't damage the tree when you collected the wood. If you said "I used a tree branch found in the forest. But I don’t harm trees." (omitting the word the) you would be making a general statement about not harming any trees, at any time. Sorry the answer is long- I hope I have explained it clearly for you. As you can see it all depends on what you are trying to say.

yes, if you want to express two teonses, what you actually did in the past(I used tree branch found in the forest), and a fact that concerns you (But I don’t harm the trees!)...

Yupp :)

To make it grammatically correct you need to stick to one time form, either past or present, example :I used tree branches found in the forest. But I didn't harm the trees! I only picked up broken sticks fallen after stormy weather. Ingriedients: wood, nature, love."