How can I develop an own text to speech software from scratch?

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

using System.Speech.Synthesis;
using System.Speech.AudioFormat;

 public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        List<string> all_lines_List = new List<string>();

        private void Read_Click(object sender, EventArgs e)
        {
            if(comboBox1.SelectedItem!=null)
            {
                button1.Enabled = false;

                
                SpeechSynthesizer speech_synthesizer = new SpeechSynthesizer();

                ReadOnlyCollection<InstalledVoice> InstalledVoices = speech_synthesizer.GetInstalledVoices();
                


                speech_synthesizer.SelectVoice(comboBox1.SelectedItem.ToString());
                //speech_synthesizer.SelectVoice("MSMary");  
               
                speech_synthesizer.SetOutputToDefaultAudioDevice();
                //PromptBuilder builder = new PromptBuilder();
                //builder.AppendSsmlMarkup("<say-as interpret-as = \"chs\"> chair </say-as>");

                //speech_synthesizer.AddLexicon(new Uri("C:\\W7\\Spelling.pls"), "application/pls");
                
                speech_synthesizer.Volume = 100;
                speech_synthesizer.Rate = 0;

                PromptBuilder builder = new PromptBuilder();
                builder.AppendText("This is sample output to a WAVE file.", PromptEmphasis.Strong);
                builder.AppendSsmlMarkup("<say-as interpret-as = \"WAVE\"> chair </say-as>");

                speech_synthesizer.Speak(builder);

                speech_synthesizer.Speak(richTextBox2.Text);

                //speech_synthesizer.RemoveLexicon(new Uri("C:\\W7\\Spelling.pls"));
                button1.Enabled = true;                

                System.Media.SystemSounds.Asterisk.Play();

                Application.DoEvents();
            
            }
            else
            {
                System.Media.SystemSounds.Hand.Play();
                MessageBox.Show("Please, Select a Voice.");
            }


           
        }

        private void Save_Without_Reading_Click(object sender, EventArgs e)
        {
                      
            if (comboBox1.SelectedItem != null)
            {
                SpeechSynthesizer speech_synthesizer = new SpeechSynthesizer();

                speech_synthesizer.SelectVoice(comboBox1.SelectedItem.ToString());
                
                speech_synthesizer.SetOutputToDefaultAudioDevice();

                speech_synthesizer.Volume = 100;
                speech_synthesizer.Rate = 0;

                speech_synthesizer.SetOutputToWaveFile(comboBox1.SelectedItem.ToString() + " - Speech.wav", new SpeechAudioFormatInfo(44100, AudioBitsPerSample.Sixteen, AudioChannel.Mono));

                speech_synthesizer.Speak(richTextBox2.Text);
                System.Media.SystemSounds.Asterisk.Play();                

            }
            else
            {
                System.Media.SystemSounds.Hand.Play();
                MessageBox.Show("Please, Select a Voice.");
            }

            
        }