Generally, commands (one line per command) can be added or deleted using Add / Delete buttons on the script panel, or through Right Click Menu on lines of script. With minor difference, it's mostly a matter of user preference.
The Add tool button adds a new command on a blank line at the end of the script. This is the only method for starting out a blank script, as there are not yet any command lines to right click upon.
The Insert and Delete tool button functions depends upon one or more lines of script being highlighted. If there are no lines highlighted it will tell you so. If there are lines highlighted Delete will remove them, while Insert will insert new command lines in front of the highlighted lines, the number of new lines matching the number highlighted.
Once a command line is added to the script, it can be turned into any available command by right-click selection. Right click selection can also choose to turn the command into a blank line, or insert a new command or blank line in front of it.
Blank lines are described as comment on the right click menu. Actually comments can be added in the last column on any command line, although blank lines can also serve both adding comments and visually separating groups of commands.