A surgical instrument assembly for use during a surgical procedure to implant a stemless humeral component to replace the humeral head of a patient's humerus, comprising: a sizing instrument having a size that mimics the size of the stemless humeral component to be implanted to replace the humeral head of the patient's humerus, the sizing instrument having a generally round body with a plurality of punch guide holes formed therein, wherein each of the plurality of punch guide holes is positioned in a location that (a) is located over cancellous bone tissue of the patient's humerus when the sizing instrument is secured to the patient's humerus, and (b) aligns with a plurality of legs of the stemless humeral component, characterised in that the generally round body of the sizing instrument is generally dome-shaped having a flattened upper surface and a planar opposite, lower surface, and an elongated drill guide bore extends through a centre of the sizing instrument from the flattened upper surface to the lower surface, and the drill guide bore is positioned in a location to engage the cancellous bone tissue of the patient's humerus when the sizing instrument is secured to the patient's humerus, the surgical instrument assembly further comprising: a trial head component, in which: the trial head component comprises (i) a generally hemispherically-shaped trial bearing surface, (ii) an underside surface opposite the trial bearing surface, wherein the diameter of the underside surface of the trial head component is substantially the same as the diameter of the sizing instrument, and (iii) a centre lug extending laterally away from the underside surface, the centre lug being configured to be frictionally secured to the sizing instrument when the centre lug is inserted into the elongated drill guide bore. a sizing instrument having a size that mimics the size of the stemless humeral component to be implanted to replace the humeral head of the patient's humerus, the sizing instrument having a generally round body with a plurality of punch guide holes formed therein, wherein each of the plurality of punch guide holes is positioned in a location that (a) is located over cancellous bone tissue of the patient's humerus when the sizing instrument is secured to the patient's humerus, and (b) aligns with a plurality of legs of the stemless humeral component, characterised in that the generally round body of the sizing instrument is generally dome-shaped having a flattened upper surface and a planar opposite, lower surface, and an elongated drill guide bore extends through a centre of the sizing instrument from the flattened upper surface to the lower surface, and the drill guide bore is positioned in a location to engage the cancellous bone tissue of the patient's humerus when the sizing instrument is secured to the patient's humerus, the surgical instrument assembly further comprising: a trial head component, in which: the trial head component comprises (i) a generally hemispherically-shaped trial bearing surface, (ii) an underside surface opposite the trial bearing surface, wherein the diameter of the underside surface of the trial head component is substantially the same as the diameter of the sizing instrument, and (iii) a centre lug extending laterally away from the underside surface, the centre lug being configured to be frictionally secured to the sizing instrument when the centre lug is inserted into the elongated drill guide bore.