They are the easiest canisters to prime IME.
If you don't pre-fill the canister, you will need to remove the bottom off the outlet duck bill as the priming plunger draws water into the filter through the outlet. By priming the plunger, water will be drawn into the outlet pipe and you will need to top up the tank. IME, it will take about three or four strokes of the plunger before the syphon effect takes over and fills the canister. Once the water level in the tank stops dropping, the canister is full and you can put the bottom plug back on the duck bill and switch on. You will need to make sure that the bottom of the duck bill stays submerged or the syphon will stop.
The above does not work if you have a spray bar fitted so you will need to pre-fill the canister. Fill up to the top of the baskets and close the lid. Attach the pipes and prime the plunger a couple of times and switch on.
With either method, you may notice a noise from the impellor when you switch on. This will disappear after a few seconds once the canister is 100% full.
If the plunger does not seem to draw water, it may need 'refreshing'. To do this, pull off the silicone cup from inside the powerhead and drop it into boiling water for about a minute to soften the silicone rubber. Then press it firmly onto a smooth surface to spread it's shape. This will return the cup to it's proper shape and you can re-fit it.