This is such a broad question, it's actually hard to answer.

Start by pre-treating the waste oil.

The classic approach is to do a Fischer Speir esterification (acid catalyzed) on the FFA's (free fatty acids) followed by a base-catalyzed transesterification (ideally with dry NaOMe or KOMe or the like). This is a rock solid approach and scales well to millions of gallons/yr.

If it's more researchy and less industrial - microwave assisted esterification is quick and versatile.

If it's more data driven - monophasic approaches (e.g. with THF cosolvent) are FAST, but hampered by water even more.

That's probably enough to get started down the right track.