Microphones are an important but expensive part of a studio setup so making your own might seem like a tall order. In this tutorial video, youtuber Sound Sleuth runs us through making his own OPA condenser mics from his extensive research and experience. The two mics he builds use TSC-1 and TSC-2 capsules, similar to the Neumann K87 which is in their U87 microphone.
The whole build process is detailed in an Instructables tutorial with accompanying theory, list of materials and construction details. The TSC-2 capsule used is able to receive signals from both sides and in his design both signals are sent out of the mic rather than being set internally - allowing for the mic response to be changed to omnidirectional, cardoid, or figure-of-eight. More information and background to the project here.