Generation 7 Electronics is designed to be as simple and as dummy-proof as possible. This led to the decision to use ATX power supplies, which can be plugged in as-is. No wiring, no soldering, no crimping neccessary. Just put the plugs where they fit until all three headers are filled: ATX24 with 24 pins, ATX12V with 4, 6 or 8 pins and one Disk Power connector.
Using an ATX power supply you're already done here.
Symptom: the PSU turns off as soon as a heater or a stepper is turned on.
Discussion: see ATX Power Supply, "Base Load".
Note: this applies only if when an ATX PSU is to be avoided.
One of the most often heard complaints against ATX supplies are that they're larger. That's true, 12-V-only supplies of the same size are typically smaller.
A not exactly cheap and also not strong enough for a heated bed, but a simple to apply solution are “Pico” power supplies. Like this Pico PSU. They're made for ITX boards and come with all the neccessary connectors. They also need an additional external power supply.
Note: this applies only if when ATX PSUs are to be avoided.
One can replace all three connectors with screw terminals. After removal of the genuine headers, the most often used ones with 5.0 or 5.08 mm pin spacing fit into the existing holes.
Then, the 3.3 volts supply has to be adjusted. To be continued.