To pitch an adequate amount of yeast in your wort, you should always prepare a starter.
In professional brewing, a count of 3 million cells per ml for every 4 °Plato is a standard for an ale yeast, while a bottom fermenting yeast would require twice as much.
In my experience, pitching 1 liter of actively fermenting and highly viable starter provides a count of about 5 million cells per ml, which is considered acceptable for homebrewing purposes, leading to a lag time of less than 6 hours. For a high gravity ale or a lager, you need 2 liters of starter.
If you own a microscope and a hemacytometer, you can perform an accurate cell count, but this is not strictly necessary.
When you acquire a new yeast strain in a Wyeast pack, you will pitch about 50 ml of active culture in a flask with 250 ml of sterile wort.
In other situations however, a few more steps are required.
If you are starting from a master stored in distilled water, I suggest you first plate it, then use solid to liquid sterile transfer. Or you can skip the plating and use liquid to liquid sterile transfer.
If you are starting directly from a plate or slant, use solid to liquid sterile transfer.
In either case you will end up inoculating a culture tube containing 10 ml of sterile wort.
Incubate this tube in a warm (>27 °C) dry clean place. Expect signs of fermentation within 12 hours. Periodically crack the cap open and swirl to release CO2.
As soon as fermentation begins, you should transfer the content of the tube to a small bottle or Erlenmayer flask containing 50 ml of sterile wort. This larger container should have a screw cap, rubber closure, or glass cap.
Prepare the flask using the wort to flask sterile transfer technique.
The flask now contains about 50 ml of sterile wort.
Now you can transfer the content of the culture tube into the flask, using liquid to liquid sterile transfer.
Incubate the flask in a warm (>27 °C)dry clean place. Expect signs of fermentation in a few hours. Periodically crack the cap open to release CO2.
As soon as fermentation begins, you should transfer to a larger flask (500 ml) containing 250 ml of sterile wort.
To prepare this larger flask, again use the wort to flask sterile transfer technique.
Then transfer the contents of the 50 ml flask to the 500 ml flask using liquid to liquid sterile transfer.
Having now a flask with approximately 300 ml of fermenting starter, you proceed with the final step as explained later.
Smack the pack as usual and incubate in a warm environment. When the pack swells to about 3 cm thickness, but before it becomes a balloon, you should draw a sample for selection and storage, and pitch the rest in your starter.
The usual precautions apply: scrub the table surface, test tube, Wyeast pack, scissors and your hands with alcohol. Wear a mask, keep your work area as dust-free as you can, work in flame zone.
Following the wort to flask sterile transfer technique, prepare a 500 ml flask containing 250 ml of sterile wort.
Next, have everything ready on the table: test tube with liquid media (sterile wort) standing in its rack, syringe, scissors, a sterile 500 ml flask with cap, containing 250 ml sterile wort.
When ready, proceed as follows:
Now you have a culture tube inoculated with fresh yeast. Incubate it in a warm (>27 °C) dry clean place. Expect signs of fermentation within 12 hours. Crack the cap open to release CO2, then proceed with plating for selection and storage.
You also have a flask with 300 ml of starter. This also should show signs of fermentation within a few hours. As soon as it's fermenting, step it up to a larger flask containing 700 ml of sterile wort.
First you have to prepare a 1000 ml flask containing 600 ml of sterile wort.
To do this, follow the now familiar wort to flask sterile transfer technique.
Then transfer the contents of the 500 ml flask to the 1000 ml flask using liquid to liquid sterile transfer.
When this last step is actively fermenting, you are ready to pitch into your wort.
If you are worried about introducing a large quantity of starter wort in your batch, you can allow the last step to ferment almost out, discard most of the liquid portion, and add fresh wort after the boil and chilling. By the time you have racked to primary, removed trub, aerated, etc. you should be ready to pitch.
For a lager or a very high gravity beer, the common procedure is to double the last step: after you have 300 ml of starter, transfer it in 2 flasks with 700 ml of sterile wort each. This won't require any extra time, only more wort and another flask, but you will end up with twice as much yeast to pitch.