To build an FT8 modulator I need to resume the FT8 message format. All the details come from FT4_FT8

The 174 bits message

A single 174 bits TX message is composed by:

• 77 bits the message content
• 14 bits Cyclic Redundancy Code 
• 83 bits Forward Error Correction

The 81 symbols

• The 174 bits are grouped 3 by 3 obtaining 58 symbols
• Each symbol lasts 0,16 seconds
• The 58 symbols are interposed (beginning, middle and end) by 7 grey coded symbols to ease synchronization
• a single initial symbol (3) increases the amplitude with a ramp up to max amplitude
• a single final symbol (2) decreases the amplitude with a ramp down to 0

The total message lasts 160 milliseconds x 81 symbols = 12,96 seconds

8FSK modulation

Each symbol, which lasts 160 milliseconds, generates 1 of 8 frequencies shifted by 6,25 Hz i.e.

• sym: frequency shift 
• 0    : 0 Hz
• 1     : 6,25 Hz
• 2    : 12,50 Hz
• 3    : 18,75 Hz
• 4    : 25 Hz
• 5    : 31,25 Hz
• 6    : 37,50 Hz
• 7    : 43,75 Hz 

The frequency doesn't shift immediately between a symbol and the next, but it is smoothed with a Gaussian function (see Figure 2 in the article FT4_FT8).