Need help with making a diagram

Question

Please help me with making the following diagram for my presentation:

Please help. Thank you very much.

Answer 1

It's easier with tikz-cd.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz-cd}

\begin{document}

\begin{equation*}
\begin{tikzcd}[column sep=huge]
A \arrow[r] \arrow[rr,bend left] &
C \arrow[r] \arrow[dr] &
E \\
B \arrow[u] \arrow[ur] \arrow[r] &
D \arrow[ul] \arrow[u] \arrow[ur] \arrow[r] &
F \arrow[u] \arrow[ll,bend left]
\end{tikzcd}
\end{equation*}

\end{document}

You just specify the target cell with r (right), l (left), u (up) or d (down). The bending is to the left in both cases, because you should think to arrows like rivers and look from the source in order to distinguish between left and right.

Check the package manual for more information.

Answer 2

The basic concept of this site is to provide help when OP shows some effort, it is special true for not simple tasks. As your are looking for the basics, and I usually draw many block diagrams like yours, I then provide a simple and quick answer below.

Using tikz, first define all nodes you need, then connect them using the arrow with your favorite style, in this case, I used mySimpleArrow defined with \tikzset. To draw curved lines, use edge[bend <side>=<degree>].

\documentclass[border=5pt]{standalone}
\usepackage{tikz}
\tikzset{mySimpleArrow/.style n args={2}{
    >={latex[#1]},
    every path/.style={draw=#2}
  },
  mySimpleArrow/.default={black}{black}
}
\begin{document}
\begin{tikzpicture}[very thick]
  \node (A) at (0,0) {A};
  \node (C) at (3,0) {C};
  \node (E) at (6,0) {E};
  
  \node (B) at (0,-1.5) {B};
  \node (D) at (3,-1.5) {D};
  \node (F) at (6,-1.5) {F};
  
  \begin{scope}[mySimpleArrow]
    \path[->] (A) -- (C);
    \path[->] (A) edge[bend right=-30]  (E);
    
    
    \path[->] (B) -- (A);
    \path[->] (B) -- (C);
    \path[->] (B) -- (D);
    
    \path[->] (C) -- (E);
    \path[->] (C) -- (F);
    
    \path[->] (D) -- (A);
    \path[->] (D) -- (C);
    \path[->] (D) -- (E);
    \path[->] (D) -- (F);
    
    \path[->] (F) -- (E);
    \path[->] (F) edge[bend right=-30] (B);
  \end{scope}
\end{tikzpicture}
\end{document}