Fill a blender with 400ml of cold water, 1 peeled lemon, 1 peeled large orange and approximately 50g of fresh or frozen pineapple. Then blend for 2 mins. Pulse further if you see bits in your blender.
Take your sieve and place it over a large bowl. Gently pour in your blended mixture into the sieve.
Using a spoon or a spatula, slowly move the mixture in the sieve so that the juice is able to trickle into your bowl. You may need to do this multiple times, in small batches, to use up the liquid mixture, depending on the size of your sieve.