The Scottish sportsman is one of several personalities who will be honoured in the ceremony on Thursday, when William stands in for his grandmother the Queen. In doing so, the 31-year-old is taking his first major step towards being a working royal.
A royal source said: "William has been practising using the sword and pinning the medals on a servant so that he gets it right.
"He obviously doesn't want to slice anyone's ear off or stick a pin into their chest!"
The Prince will dub with a sword to create two new knights and will also pin a medal onto Vicar of Dibley producer Jon Plowman."
He realises this is the biggest day in the lives of some people who are being honoured, and he wants it to go well for them," continues the royal aide.
"His biggest challenge is being tall – it's not easy to stoop down and talk to some of the smaller recipients and to pin the honours on."
The Queen is understood to have asked William several months ago if he would start performing investitures, to which he immediately agreed.
The Prince – who left operational military service in September – wants to ease slowly into increasing his royal duties, so he does each one well without rushing into his new role as a working royal.
William and Kate have supported tennis ace Andy during several important matches during his career.
But the Duke will have to resist temptation to stay chatting to the 26-year-old sportsman after pining on his medal – he must keep to the allotted time of 30 seconds conversing with each recipient to avoid the ceremony overrunning.
He has a choice of three uniforms – army, navy and air force – to choose from for the hour-long ceremony. Prince Charles – who regularly stands in for his mother – usually wears his Royal Navy suit.
Thursday's ceremony is one of around 25 which are held annually at venues including Buckingham Palace, Windsor Castle, Holyroodhouse in Edinburgh and sometimes even abroad.