Couple of quick tips:
1. Find a good recipe or two, there are loads online. My favorites are from a nectar-making sim called Carmen Angelos. You can experiment instead, of course, but it can be quite frustrating.
2. Grow a lot of life fruits, and get them to perfect. Most of the high-value nectars are based on life fruits.
3. Use the nectar racks from France to store your nectar in a basement, think (at least slightly) long-term.
4. Use a dedicated "consigner sim" to consign your nectar at the consignment shop, to get a good reputation and increased value for your nectar. Doesn't hurt if he/she has the LTR "Suave Seller".