\documentclass{standalone}
\standaloneconfig{margin=1cm}
%\usepackage[margin=0.5cm]{geometry}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric, arrows, positioning}

% Define styles
\tikzstyle{startstop} = [rectangle, rounded corners, minimum width=2cm, minimum height=1cm, text centered, draw=black, fill=red!30]
\tikzstyle{process} = [rectangle, minimum width=3.5cm, minimum height=1cm,text width=7cm, text centered, draw=black, fill=blue!20]
\tikzstyle{decision} = [diamond, minimum width=2.5cm, minimum height=1cm,text width=3cm, text centered, draw=black, fill=green!30]
\tikzstyle{arrow} = [thick,->,>=stealth]

\begin{document}

\begin{tikzpicture}[node distance=2cm, scale=0.5]

% Nodes
\node (start) [startstop] {Début};
\node (step1) [process, below of=start] {Écrire le système sous forme standard};
\node (step2) [process, below of=step1] {Construire le premier tableau correspondant à la forme standard};
\node (step3) [process, below of=step2] {Choisir la variable à introduire dans la base};
\node (step4) [process, below of=step3] {Choisir la variable à enlever de la base};
\node (step5) [process, below of=step4] {Encadrer le pivot};
\node (step6) [process, below of=step5] {Diviser la ligne du pivot par le pivot};
\node (step7) [process, below of=step6] {Calculer les valeurs des autres lignes};
\node (decision) [decision, left of=step5, xshift=-4.5cm, ] {Les coefficients de la fonction objectif sont-ils tous nuls ou négatifs ?};
\node (stop) [startstop, left of=decision, xshift=-2.5cm] {Fin};

% Arrowsh
\draw [arrow] (start) -- (step1);
\draw [arrow] (step1) -- (step2);
\draw [arrow] (step2) -- (step3);
\draw [arrow] (step3) -- (step4);
\draw [arrow] (step4) -- (step5);
\draw [arrow] (step5) -- (step6);
\draw [arrow] (step6) -- (step7);
\draw [arrow] (decision.south) -- ++(0,-2) |-  (step7.west);
\draw [arrow] (decision.north) -- ++(0,3) |- (step3.west) node[midway, left] {Non};
\draw [arrow] (decision.west) -- (stop.east) node[midway, above] {Oui};

% Step labels
\node[right=0.15cm of step1] {Étape 1};
\node[right=0.15cm of step2] {Étape 2};
\node[right=0.15cm of step3] {Étape 3};
\node[right=0.15cm of step4] {Étape 4};
\node[right=0.15cm of step5] {Étape 5};
\node[right=0.15cm of step6] {Étape 6};
\node[right=0.15cm of step7] {Étape 7};
\node[above left=0.3cm and 0.2cm of decision] {Étape 8};

\node[left=of start,xshift=-2cm] {\textbf{\textcolor{blue!60}{\Huge Algorithme du Simplexe:}}};

\end{tikzpicture}

\end{document}