If you really want to get the best out of your men, unleash them on an unsuspecting British countryside with no money, a list of items they have to beg, borrow or steal and a hunter force of police and Home Guard on their tail.
That was the no-holds-barred approach of SAS commander Lieutenant Colonel Robert Blair "Paddy" Mayne as he prepared his soldiers for Operation Gain (1944), a series of key sabotage missions into occupied France in the run-up to the D-Day landings.
A fascinating new book by Sunday Times bestselling author Damien Lewis reveals how this was one of the first training missions set for one particular team of SAS raiders - SABU-70, a 12-man team tasked with sabotaging transport routes in northern France.
This baptism of fire started in east Ayrshire on a 400-mile journey to Chelmsford, checking in at a variety of points en route without being caught.
By the time SABU-70 reached their goal, beating the other SAS teams, they had tied up a squad of constables who had tried to apprehend them for poaching, stolen their police car, hijacked an Army truck and stolen a double-decker bus before cadging a lift on a Churchill tank and a furniture lorry.
These antics didn't go down well with the spit and polish brigade within the Army, but as soon as SABU-70 came in victorious, they were dispatched with the rest of the SAS to a secret base in southern England to deploy into Nazi-occupied Europe as part of Operation Gain.
From there, they embarked on a series of daring missions aimed at hamstringing the Nazi response to the D-Day landings, only to fall at last into an enemy trap, undergoing torture at the Gestapo's Parisian headquarters and swept off under cover of darkness to face an illegal firing squad which only a few survived.
Decades later the story of their feats has finally been put to paper thanks to the painstaking research of former war reporter Lewis, and the fate of that 12-strong squad, including five from Northern Ireland, makes for a bittersweet tale.
SABU-70 was a 12-man patrol led by war hero Captain Patrick Garstin, whose family was of Irish origins and whose father had been born in Randalstown.
"Mayne did favour Irish recruits, so Garstin was welcomed with open arms and put in charge of the 11 men of SABU-70, which probably stood for Save All, Business As Usual," Lewis explains. Garstin divided his squad into two "sticks", each of six men, with all the Irish soldiers under his personal command.
Among this motley crew were "restless soul", Lurgan-born Lance Corporal Howard Lutton, who had falsified his age in order to sign up for the Royal Ulster Rifles, and Cookstown-born Tom "Paddy" Barker, known as Tot, who had signed up with the Royal Inniskilling Fusiliers before volunteering for the SAS aged 20.
Co Down farm labourer Trooper Joe Walker had signed up for the Royal Ulster Rifles, and spent considerable time in the brig.
But once he'd volunteered for the SAS and been sent for airborne training, his "cheery disposition" and hard-working nature won through.
Meanwhile, Trooper Billy Young, the eldest of five sons from Co Antrim, had been a dairyman by trade before joining the Royal Ulster Rifles.
He volunteered for 1 SAS on the same day as Walker.
"They were a very colourful cast of characters and when you read about their records, you can see that some had been promoted and demoted in as many months as they had been in the military," Lewis says.
"Mayne welcomed people from Ireland north and south, Catholic or Protestant, it didn't matter to him - the more fighting Irish he could have under his command, the better."
Thanks to a wealth of previously unavailable material from private archives, Lewis has pieced together the tale of the triumphs and tragedy of this elite band of soldiers, not least the horrific trap that led them into the hands of the ruthless Gestapo.
"The German military were getting more and more annoyed that they couldn't track down those SAS raiders who had linked up with the French Resistance to multiply their numbers," says Lewis.
"They were incensed that they couldn't find the SAS headquarters in the deep forests and they carried out horrific reprisals.
"Hitler viewed the special forces operations as a personal insult against him and the high command."
Hitler's Commando Order, issued in October 1942, stated that all Allied commandos would be killed immediately and without trial, even if they were in their uniforms or attempted to surrender.
"They would be kept alive only long enough to be interrogated and then shot out of hand," Lewis says.
"They should have been afforded all the protections of the Geneva Convention if captured, but in Hitler's phrase: 'They will disappear into the night and the fog and even their nearest and dearest will never find out what happened to them.'"
On the night of July 5, 1944, Garstin's team parachuted into a cornfield in the forest of Fontainebleau and were immediately ambushed by the waiting Gestapo, who managed to capture seven of the parachutists.
Lieutenant John Wiehe, Paddy Barker, Sergeant Thomas Varey and Corporal Serge Vaculik had all been wounded by gunfire, and Howard Lutton was fatally injured.
"They were taken to Avenue Foch, the Gestapo's headquarters in Paris, where they were tortured and interrogated, and eventually Hitler himself issued the order that they were to be executed," Lewis says.
"In early August they were forced to change into civilian clothes under the cover story that they were to be exchanged for German soldiers, driven to a patch of woodland north of Paris and all seven faced an execution squad." Realising the cover story was a lie, the badly injured Captain Garstin warned his comrades to make a break for it while he distracted his captors.
Corporal Ginger Jones and Vaculik escaped, going on to soldier with the French Resistance before rejoining the SAS and helping to bring to justice the Gestapo war criminals responsible for killing their comrades.
The latter part of the book delves into the actions of the newly formed SAS War Crimes Investigation Unit, which tracked down and brought to justice the war criminals who had been active in occupied Europe.
Although the SAS was disbanded in autumn 1945, the team, known as the Secret Hunters, continued to operate covertly with the backing of Churchill, who had massaged the security budget to hide its existence.
Lewis himself is no stranger to the conflict zone. Now 54, a father-of-four and living in Dorset with wife Eva, he cut his teeth on a Land Rover drive to the Congo on a Winston Churchill Fellowship and made an award-winning documentary on the rainforest, followed by a filming trip to the world's worst warzone in Burma (now Myanmar), embedded for a year with the Karen tribe, who were fighting the Burmese military.
What particularly resonates with the release of SAS: Band Of Brothers is the way Lewis has been able to work so closely with the families of many of the key protagonists, including Fiona Ferguson, the niece of Blair Mayne, who opened the family archive and read Lewis's manuscript in draft form.
Much of the story came from Sean Garstin, who was just one when his father Captain Garstin was killed, yet he also discovered much he hadn't known.
"He said he had learned so much about his father that he'd never known, from reading the book," Lewis says.
Meanwhile, the writer has been in contact with the grand nephew of Howard Lutton, who was the first to die in the ambush.
"One of the men was Paddy Barker, who was injured during the ambush, taken to hospital and then to his execution. I was contacted by the nephew of one of the Irishmen who signed up with him - Paddy Barker's best friend, who is still alive. Both of them lied about their ages to get into the military."
SAS: Band Of Brothers by Damien Lewis is published by Quercus Books, priced £20