How to make table legs from wooden planks?
90 degree planks will be plenty strong assuming they are not super narrow. Without knowing the details, it's not possible to suggest the best connection. You will want to connect to whatever member has the greatest vertical depth for maximum stability. Do everything possible to ensure the legs are rigidly attached. Nails or screws alone may not be enough. If possible, slightly rabbet the legs into something. The rabbet does not need to be very deep to help significantly. Of course, the fit needs to be fairly snug to be of benefit. Also glue the joints unless the need for disassembly is important. Common woodworker's glue is adequate.