Looking at the process of commissioning U-570 as HMS Graph can give us some understanding of how this process worked. Following its capture, the Royal Navy spent about a month repairing it, working out how it worked, and bringing her into service.

Some of its systems were easily understandable, because they were analogous to systems on the RN's own submarines or other ships. Some of the basic controls, like the steering control or periscope, would have been easy to understand for a British submariner, due to understandable similarities as form followed function. While some of the dials and indicators had been damaged (either by depth charges or sabotage before the boat was captured), those that were intact could still be read by the British. Other systems had less direct analogies. Graph's electrical system, for example, worked on similar principles to equivalent British submarines. However, the more precise details of the circuitry to supply power to its auxiliary systems were similar to those fitted to smaller British surface ships. British reports on Graph noted a similarity between the torpedo mechanisms on both British and German subs. They were largely operated in the same way, with differences coming only towards the end of the firing process. These similarities, in concept or in practice, made it easier for the British to understand how the submarine worked. They weren't starting from a blank slate, but could apply knowledge they already had.

Another aid was captured materiel. When the British captured U-570, its crew had not destroyed or thrown overboard its full complement of technical manuals. Those that hadn't been destroyed included a set of plans of the boat. These were useful for repairing the damage done to it prior to its capture. More important was a booklet of diagrams of its systems. Translating these schematics gave the RN an understanding of how these systems were laid out. Experienced submarine officers analysing these schematics could also figure out how they worked. The RN's reports also mention a booklet of general instructions, produced by Akkumulatoren Fabrik A.G, for the battery system. I don't know if this was captured with the boat or elsewhere. This booklet told the RN how to charge and use the batteries to their greatest effect. Other captured documents gave details on the operation of the engines. Unofficial materials were also captured with the boat. The engineering crew had notebooks which contained some detail on the sub's construction and systems. These could reveal useful or significant information on the operation of the boat - the procedures used by the crew, for example. Materiel captured with other boats could also be applied to U-570, but I don't know if any were used here.

Interrogations of the crew could also give information. This was limited, as the RN did not want to reveal that the sub had been captured intact and was being used. They couldn't take a captured sailor into the boat and ask him 'what does this switch do', as to do so would reveal that they had it in their possession. However, they could ask about what systems were fitted. The British tended question prisoners of war shortly after they entered custody, before they were sent to their final POW camp. Interrogations of captured sailors from U-570 gave information about how its hydrophone and radio systems worked. Interrogations from POWs from other boats might also give some information that was useful on U-570.

Finally, there was a degree of trial and error that went into working things out. The British could try operating an unknown control, and record what the result was. Doing this systematically would give a full understanding of the boat's operation. However, this was risky, as careless operation of systems could result in damage to the boat, injuries to her crew, or even the loss of the boat. During an early attempt at testing its cooling system, for example, Graph nearly sank while alongside the depot ship Hecla. It turned out that the Germans had removed the cover to an inlet valve, allowing the boat to flood uncontrollably, an attempt at sabotage that had not been caught by the initial British inspection.