If you have already tried to use citric acid as a softener (recipe here) you probably have realised that it is a solution that has no perfume.
Personally, I don't mind if my washed clothes lack a fragrance and I don't like chemically-perfumed garments, because the "fake good smell" of the fabric conditioner reminds me how bad it is for us and the environment.
In fact, commercial softeners contain, in order to perfume, very polluting molecules that persist in the environment, stick on your garments and come in contact with your skin as well.
But I can entirely understand that you want your laundry to smell good, especially in case of the dark synthetics fibres that, sometimes, just after washed, release a terrible odour.
Some people add drops of essential oil directly to the citric acid solution, but I don't find this idea very useful.
Instead, a good alternative is to make a laundry perfume.
I prefer working in grams because it is more accurate than ml/tbs/oz.
200-250 g of Sodium Bicarbonate
or (even better) Cornflour in a jar
How to prepare
Add some drops of your favourite essential oil in the jar with the cornflour, stir well and let the mix sit overnight.
The next day it will be ready for your laundry!
How to use
Put one or two spoons of the powder directly into the washing machine drum with your clothes on the top. If the scent is not strong enough, add more drops of essential oil until you will be happy with the result!
Another traditional and very ecological remedy is to buy fragrance bags and leave them in the drawers between your clothes. They usually are made of dried lavender blossoms, and they don't just smell lovely, but are perfect as a natural moth repellent.
You can also pour a few drops of your favourite essential oil on a piece of paper and put it in your drawers and wardrobes.