I just searched, and the Wikipedia page on USB contains some basic information. If the connector is facing towards you and the clips to secure the cable are facing down, then the pins are numbered 1-5 from left to right. Pin 4 is the "ID" pin which is used to detect if the A or B type connector is inserted, and pin 5 is the ground. In an A type connector those two pins are bridged, so you'd want to short those two pins together if you want the device you plug it in to see your type B cable as if it has a type A connector (the connector is a slightly different shape, but that doesn't matter, it'll still fit and the ID pin is the only method used to detect the connector type).

If you short the pins together by inserting something conductive inside the connector then you'll likely also short them to the case, but that shouldn't matter as the case is internally almost always connected to the ground too.

If you take the route of soldering two cables together, I looked into it a bit more and it's apparently a bigger mess than I anticipated. Hardware manufacturers have always treated the USB specs as more of a suggestion than as fixed standards, and apparently this is by far the worst when it comes to micro USB. Most phones and tablets with micro USB which support OTG use the type B receptacle instead of the AB receptacle, even though according to the standards only the AB receptacle is allowed for a device that can act as the host. The majority of micro USB OTG cables are then designed for such devices and use a type B connector for the host side. So if you use an OTG cable to harvest the micro-A connector from, make sure you get a standards-compliant cable. You can see the difference by the shape: for the proper micro-A connector the metal casing on the outside is rectangular in shape, while for the much more common micro-B connector it is roughly trapezoidal. According to the standards the plastic inside the connector should also be white for a type A connector (and black for type B), but those colours are probably the most often violated part of the standards.