I have bought an MCB which is all I need but the installation
instruction does not show how to wire the box.

I'm afraid that I'm slightly worried by this question. A competent person
should be installing the consumer unit. With this in mind, they are
sufficiently simple not to warrant installation instructions, really.

Also, if such work was occuring, it would be an excellent time to get the
dangerous old wire fusebox replaced by a large modern consumer unit.

Christian.