The aim when playing is to make a vocal sound, achieved by a steady and well-controlled stream of air. Breathing appropriately is an acquired skill, the aim being to use a large capacity of the lungs supported by the diaphragm. Maintaining a steady sound across the whole range of the recorder takes practice. The upper octave will naturally be louder than the lowest notes if the same breath pressure is maintained, and players should try to keep the bottom range solid with slower air, and the upper range light with a quicker, narrower air stream. A common mistake is to overblow notes to obtain the higher octaves, whereas they should be reached by partially opening the thumbhole most efficient with a carefully maintained thumbnail.
Co-ordination of fingers and tonguing articulation is equally important, and exercises can improve this. In terms of posture, players should never grab the recorder tightly, but keep the fingers loose, although with the instrument still feeling secure and well-balanced. Squeaks or uncentred notes are usually due to fingers allowing air to leak practising in front of a mirror or playing to someone else can help to detect such errors. Another hurdle is a reluctance to lift the fingers away from the holes - try imagining springs on all the knuckles efficiently raising the joints.
More modern compositions can make other technical demands, such as flutter-tonguing, multiphonics (certain fingerings produce two notes simultaneously when overblown), extremely high notes, finger vibrato, and even tapping the fingernails against the side of the instrument while playing all great fun, but probably best to master the basics first!