I'm by no means an expert...but this has worked for me for a cold start: If running on the bench, choke to bring fuel to the needle. For most engines, richen up the needle about 1/2 to 3/4 turn from your optimum setting when starting cold. Then with the exhaust port closed, a few drops into the venturi, and a small squirt on the side of the piston, then start flippin'. As you flip and the piston opens on the exhaust port, it will suck in some of the ether vapor, which is what you want. The ether is what gets them going when it's cold.
If the engine is a high compression racing type, bolting on a larger diameter prop than what you fly with just to get the engine running and get some heat into it can be helpful.
When they are hot, each engine seems rather individual. Some you can just refuel and flip, others need an exhaust and/or venturi prime.
To further complicate matters, often the engine behaves completely different when in a model.
Just try a few things and eventually you'll develop a procedure.
Les