\begin{figure}
.....
\end{figure}

\framebox{x_dimension, y_dimension}[position]{text}

\documentclass[12pt]{article}
\begin{document}
\begin{figure}[h]
\setlength{\unitlength}{0.14in} % used to set the unit length
\centering % used for centering Figure
\begin{picture}(30,16) % these are the dimension of the picture environment having 32 length units width and 15 units height.
\put(0,5){\framebox(8,3){$Transformer$}}
\put(12,5){\framebox(6,3){$Rectifier$}} % the distance between the point on the x-axis is used to determine the gap between the particular block
\put(22,5){\framebox(6,3){$Smoothing$}} % the point on x-axis is the distance from the origin of a particular framebox
\put(31,5){\framebox(6,3){$Regulator$}}
\put(-3,6.5){\vector(1,0){3}}\put(8,6.5){\vector(1,0){4}} % here, the x and y-axis point in the put command is used to fix the arrow on the particular block
\put(18,6.5){\vector(1,0){4}}\put(28,6.5){\vector(1,0){3}}% spacing between the arrow and the block can be adjusted according to the x and y-axis coordinate on both the put commands
\put(37,6.5){\vector(1,0){4}} \put(-9,6.5) {$230V AC Mains$} \put(38,7.5) {$Regulated$} 
\put(38,6.5) {$5V DC$} 
\end{picture}
\caption{A Block diagram of regulated power supply} % title of the Figure containing blocks or the title of a whole diagram of blocks
\label{fig:block} % label to refer figure in text
\end{figure}
\end{document}

\documentclass[12pt]{article}
\begin{document}
\begin{figure}[h]
\setlength{\unitlength}{0.15in} 
\centering 
\begin{picture}(30,16)
 
\put(0,5){\framebox(5,3){$A$}} % the place of the block is at the origin
\put(10,8){\framebox(5,3){$B$}}
\put(10,2){\framebox(5,3){$C$}} 
\put(20,13){\framebox(5,3){$D$}} 
\put(20,8){\framebox(5,3){$E$}} 
\put(20,4){\framebox(5,3){$F$}}
\put(20,0){\framebox(5,3){$G$}} 
\put(28 ,6){\framebox(5,3){$H$}} % here, the position of block finishes 
\put(-3,6.5){\vector(1,0){3}} % from here the location and distance of arrow are specified
\put(5.2,6.5){\vector(1,-1){3}}\put(5.2,6.5){\vector(1,1){3}} % here the value after the vector command, i.e., {3}, signifies the length of the arrow. The higher the value, the higher will be the size of the arrow
\put(8,3.5){\vector(1,0){2}} \put(8,9.5){\vector(1,0){2}} % the position of points is adjusted according to the distance from the origin, i.e., block A
\put(15,3.5){\vector(1,0){3}} % arrow commands from block C
\put(18,3.5){\vector(1,-1){2}} \put(18,3.5){\vector(1,1){2}} % arrow commands from the arrow from block C
\put(15,9.5){\vector(1,0){5}} \put(15,9.5){\vector(1,1){5}} % arrow commands from block B
% the value of less than 1 on the y-axis in vector command is not specified here
\put(25,9.5){\vector(1,-1){3}} % arrow from block E to block H
\put(25,5.5){\vector(1,1){3}} % arrow from block F to block H
\put(33,7.5){\vector(1,0){4}}\put(-5,6.5) {$Starting$} \put(33,7.5) {$End$} 

\end{picture}\\
\caption{A simple example of a complex block diagram} 
\label{fig:block} 
\end{figure}
\end{document}