Short answer
You can train your smelling skills by regularly and consciously smelling different, clearly recognisable scents and paying attention to differences, intensity and associations. Training can mainly improve your scent recognition, discrimination and vocabulary. With medical loss of smell, a different approach and sometimes professional assessment is needed.
Smelling is partly a skill
Professional perfumers are not born with a dictionary in their nose. Expertise largely develops through repetition, comparison and learning to link scents to names and structures.
Research on olfactory expertise shows that training is associated with better performance on certain smell tasks. That does not mean that everyone will achieve an exceptionally low detection threshold through practice, but it does mean you can learn to perceive more consciously and precisely.
A simple daily exercise
Choose four to six clearly different scents. For example, lemon peel, coffee, rose, clove, cedarwood and vanilla.
Smell one scent at a time, briefly and calmly. Then try to name three things:
1. What is it?
2. How does the scent feel: fresh, dry, warm, green, creamy, sharp?
3. What does it remind you of?
Write down your answers. Compare the same scent again after a week.
Train differences, not just names
Take, for example, lemon and bergamot, rose and geranium or cedar and sandalwood. Smell them alternately and look for the difference.
This develops discrimination: the ability not just to judge two scents as "nice", but to tell them apart analytically.
Avoid olfactory fatigue
Long sessions are usually not better. After repeated exposure, the perception of a scent temporarily decreases due to adaptation and habituation.
So work in short sessions, get some fresh air and stop when everything starts to smell the same.
Smell real materials
Perfume raw materials are valuable for advanced training, but the kitchen, garden and supermarket form an excellent library. Crush a basil leaf, grate lemon peel, smell black pepper or compare different types of wood.
Medical smell training is something else
With a reduced sense of smell, medicine also uses olfactory training: repeated exposure to a set of scents over a longer period. Research shows possible benefits for various forms of smell disorder, but cause and prognosis differ. With new, severe or persistent loss of smell, medical assessment is wise.