While travelling with one bag/suitcase. What is the best way to keep dirty/used clothes in bag along clothes with clean/new clothes.
I always bring a large kitchen trash bag in my suitcase when I travel. At the end of each day, I put my dirty clothes in the trash bag (and close it, if it has a drawstring). When I re-pack my suitcase to go home at the end of the trip, I put any clean clothes in the bottom of the suitcase, then put the closed trash bag full of dirty clothes on top of it. The plastic trash bag keeps the clean clothes from getting dirty, and it also helps me unpack quickly at home: I just take the bag out and empty it right into my laundry hamper.
I use separate (ziplock) bags, for smelly/dirty clothing, for used but can be used again and for small items of clothing that tend to get lost between the bigger items.
When you do your laundry while traveling you fold the bags and store them till you get the same level of dirt again.
Instead of ziplock bags you can also use the thin nylon shopping bags or wash bags which are quite easy to get these days.
A slightly longer version on Travel Stack Exchange and the other answers to the same question might help you as well, even though the question is not a duplicate.
Use a cloth bag so the clothes can breathe. If you use a plastic bag they'll be a bit icky after a few days. If you have one with handles or a drawstring you can hang it up tidily. Those cloth shopping bags are good for this.
If you don't have a cloth bag handy use a pillow case, or a used T-shirt.