PICK YOUR SCENT
I recommend choosing a scent made with essential oils that have a therapeutic action similar to the desired emotional effect.  For instance, since lavender is known to calm and soothe, if you are hoping to achieve a sense of calm in a tense or stressful situation, choose lavender or a blend with a similar effect like Peace & Calming.
BE INTENTIONAL
Create or look for moments that match the feeling that you intend to encourage your brain to feel.  For example, if you intend to encourage yourself to feel motivated, use an invigorating scent like peppermint, or a blend like Deep Relief that has peppermint in it,  when you are feeling motivated.   Really breathe in the scent and take a moment to reflect on how good you feel at this moment.  You could even say aloud an expression of how you feel.  The goal is to create an association in your brain between the scent and the positive feeling.  Do this many times for approximately 30 days. 
USE YOUR OILS TO ENCOURAGE A POSITIVE FEELING
When you find yourself in a negative emotional state of mind, utilize the scent you have been imprinting to encourage more positive feelings.  Wear it like perfume, put it in your hands and cup your nose to breathe it in deep, and apply topically to your neck and brain stem.  Take a moment and think about the good feelings associated with that scent and focus on them.   
BEGIN IMPRINTING A NEW SCENT
Start the process over again by introducing a new scent when you are experiencing your desired feelings.  This will give you more than one scent to use in times of need and help prevent the brain from associating your scent of choice with only the negative feelings.  I have had this happen when we used peppermint for car sickness with my daughter,  It worked like a charm but eventually, she couldn't stand the smell of peppermint because it reminded her of being nauseous, the exact opposite feeling we wanted her to have.    It is good practice to have many scents to rotate.