My advice would be:
1. Look for a good market with Limited competition to base your operations and in the beginning avoid competing with other airlines, also look at gate costs I would avoid expensive airports in the beginning such as choosing LGA instead of JFK in NY.
2. In the beginning select a single fleet type and look at used aircraft and pick the correct engines with low fuel flow. Many aircraft types share maintenance groups such as 737-300,400 & 500 so if you need extra capacity choose within the same fleet type.
3. in the beginning service can be secondary, offer buy on board instead of complementary food & services & have a more densely packed cabin.
4. If you want stronger margins reduce spare staff to 0% and reduce wages.
5. Utilise gates and A/C to their full potential by flying all 140 hours and making goo use of all slots whenever possible.