What have you attempted so far?
I can't find any C# samples, but I believe you will learn what you need from the java version of this, with a guide found here: http://j8583.sourceforge.net/guide.html
These are the two main classes you need to use to work with ISO8583 messages. An IsoMessage can be encoded into a byte array or a ByteBuffer to be written to an OutputStream, or it can directly write itself to an OutputStream. You can set and get the values for each field in an IsoMessage, and it will adjust itself to use a secondary bitmap if necessary.
So as far as I can tell, you set the fields on the IsoMessage, and it handles generating the bitmap. Looking at the summary in the .NET implementation of IsoMessage confirms this:
/// <summary>
/// This class represents an ISO8583 Message. It contains up to 127 fields
/// numbered from 2 to 128; field 1 is reserved for the secondary bitmap
/// and the bitmaps are calculated automatically by the message when it is
/// going to be written to a stream.