The first step after writing a program is to enter it into the computer: these files are known as the source code. Fortran systems do not usually come with an editor of their own: the source files can be generated using any convenient text editor or word processor.
Many text editors have options which ease the drudgery of entering Fortran statements. On some you can define a single key-stroke to skip directly to the start of the statement field at column 7 (but if the source files are to conform to the standard this should work by inserting a suitable number of spaces and not a tab character). An even more useful feature is a warning when you cross the right-margin of the statement field at column 72. Most text editors make it easy to delete and insert whole words, where a word is anything delimited by spaces. It helps with later editing, therefore, to put spaces between items in Fortran statements. This also makes the program more readable.
Most programs will consist of several program units: these may go on separate files, all on one file, or any combination. On most systems it is not necessary for the main program unit to come first. When first keying in the program it may seem simpler to put the whole program on one file, but during program development it is usually more convenient to have each program unit on a separate file so that they can be edited and compiled independently. It minimises confusion if each source file has the same name as the (first) program unit that it contains.