Method 1: Washing Towels With Vinegar and Baking Soda
Step 1: Wash the towels.
Place the smelly towels in the washing machine on the hottest setting possible. Add one cup of white vinegar and one cup of baking soda to the wash. Do not include detergent or fabric softener for now. If you don't have one of the two ingredients, you don't need to worry - just one of them will do.
Step 2: Soak the towels in the machine.
Stop the machine one hour into its cycle to allow the towels to soak in the baking soda and vinegar solution. After the hour is up, turn the machine back on to allow it to complete its cycle.
Step 3: Add vinegar and detergent.
For this step, you'll need to run the wash cycle again, however this time you should add a cup of vinegar and detergent instead. Use the same amount of detergent as you normally would for a regular load and allow the cycle to complete as usual - just add an extra spin cycle. To do the latter, you can either select "extra spin cycle" on your machine, or else you can restart the spin cycle after the first one finishes.
Step 4: Put the towels into your dryer straight away.
As soon as the second spin cycle is done, be sure to put your towels straight into your dryer. Put your dryer on the highest available setting and leave it to run until the towels are completely dry. Then, repeat the drying process for a second time.
Method 2: Soaking Towels in Detergent and Hot Water
Step 1: Add 2/3 of a cup of Oxiclean to a large bucket.
Make sure you wear gloves to do this in order to protect your hands.
Step 2: Add hot water.
Gradually add very hot water to the bucket. If your tap water doesn't run very hot, you can also try adding boiling water to the bucket instead. Tilt the bucket back and forth to ensure that the Oxiclean mixes into the water. Do this gently so you don't spill water everywhere!
Step 3: Add your towels to the bucket.
Once you've filled the bucket about halfway with hot water, you can start adding towels. Ensure that you completely immerse the towels in the water and leave them to soak for 48 hours.
Step 4: Wash the towels in the washing machine.
When the 48 hours are up, wring out the towels and put them in the washing machine. Run the machine on its hottest setting and add detergent and fabric softener. You can also add Oxiclean to the wash if you like.
Step 5: Dry the towels.
Put the towels in the dryer as soon as the wash cycle is over. Set your dryer to high heat, and leave the towels in there to dry completely. After completing this method, your towels should be like new.
Method 3: Cleaning Mildew From the Washing Machine
Step 1: Check your washing machine for defects.
A correctly-functioning washing machine should drain out all the water after a wash cycle finishes. If it isn't doing so, then this could be the cause of the mildew smell on your towels. Check your washing machine for problems, and if you find that water is pooling somewhere in it, then it would be a good idea to hire a professional to fix it. Failing that, you might need to replace it.
Step 2: Wash the gasket.
The gasket is the rubber seal inside the washer that stops water from leaking out of your washing machine. To ensure that it isn't the source of the mildew smell on your towels, wipe down the gasket with a cloth dunked in soapy water. Alternatively, spray with a mild mildew cleanser. If there are places in the gasket proving difficult to reach, use an old toothbrush. Pull back sections of the gasket to get into small rubber folds to ensure you clean it in its entirety.
Step 3: Clean the detergent dispensers.
Remove the detergent dispensers from your washing machine and scrub them with a soapy water mixture consisting of a small amount of dishwashing soap and water. Note that leftover soap residue in the detergent dispensers can make your machine smell of mildew. If it's not possible for you to remove the dispensers, do your best to clean them with a rag or pipe cleaners.
Step 4: Run a cleaning cycle.
Run your washing machine on its longest and hottest setting while empty. You might need to do this a few more times to get rid of the mildew smell. You can also add one cup of bleach or baking soda to the machine for a greater effect.
Step 5: Seek professional care.
If none of the above methods work, call a professional to come and dismantle the machine and diagnose the problem properly.
Step 6: Take preventative measures.
Once you've figured out where the smell is coming from, you should take preventative measure to ensure that mildew doesn't accumulate again. You can do this by adequately airing out your washing machine, using detergent sparingly,
replacing liquid fabric softener with dryer sheets or dryer balls and running a bleach cycle once per month.
If these techniques aren't quite what you're looking for, then don't forget to check out the vinegar and baking powder method!