Put the shirt on. Pinch the cuffs of the sleeves together. Do not overlap one over the other when doing this.
Insert the cufflink through the two buttonholes on the shirt's cuff. The decorative part of the cufflink should face the outside when the arm is at the side.
On the backside of the cuff, flip the bar to hold the cufflink in place. Repeat the process on the other sleeve.