Download and install Celtx, and run the program. The first thing you'll see is the introductory screen. Choose "Start a new project". Be sure to create a separate directory to hold your Celtx projects – we'll show you why later.

This is the main interface for Celtx. The main window in the centre is where you'll be doing most of your work – your script will appear here as you write it. The panel on the left contains different views of your work. We'll get to those later.

Celtx assumes that you want to start your script with a heading – you can see the "heading" type is selected from the drop-down box just above the main area. This is fine for us. Go ahead and type the description of your first scene, and hit "Enter". Celtx changes the paragraph type to "action", guessing (correctly) that we'll want to give a description of the scene next. Off you go, then. We'll wait.

If you hit "Enter" at the end of this paragraph, Celtx will continue to use the "action" paragraph type. This is because the program has no way of guessing what you want to do now, so it will continue happily adding action for as long as you chose to let. Add some more paragraphs of action if you like, then it's time for our first line of dialogue.

You'll notice that Celtx has chosen the "dialog" paragraph type. That seems logical – we want our character to speak after all – but sometimes you'll need to override Celtx's choices. There are two ways to do that. Either choose another option from the drop-down list, or use the tab key to cycle through the paragraph types. Hit "tab" now to change to the "paren" (or parentheses) type. Now we can add some helpful descriptive text for the actors. Hit "enter" again, and we're back to "dialog".

Type your first line, then let's have a look at the Characters tab.

You can see all the characters that have appeared in the script so far (as long as they've spoken a line) in the left-hand panel. You can manually add other characters here if you like, or delete existing characters (maybe Bobo The Clown – son of Fred – isn't an essential part of your angst-ridden space opera after all). Add another character now and switch back to your script tab. Make sure that "Char" is selected as the paragraph type, and type the first letter of your new character's name.

Celtx automatically prompts you to complete the name. Good, eh? If there's more than one character who's name begins with that letter, Celtx will let you choose from a list. Of course, you can ignore the prompt entirely and enter a new character's name (in which case, a new character will automatically be created for you in the characters tab).

What you're looking at here is a database of items that are going to be important to your script. It's pretty sparse at the moment – all the categories are empty – but we'll soon change that. Select "set" as your category, and chose "new item". Give your set item a name and a description, and hit the "tag" button. Now we have an old poplar tree in our list of set items, and the mention of this item in the script is highlighted in blue to reflect this.

As it happens, we've already chosen the model that we're going to use for our poplar tree, so let's add a picture so we don't forget. Make sure that your poplar tree is selected in the sidebar, and click the "add" button at the bottom of the "Media" section. Celtx will throw up a navigation box – find a suitable image on your hard drive, and voilà – your poplar tree has a pretty picture to back it up.

Yes, we know it's not really a poplar tree. Actually, the designers of The Movies, botanists all, describe it as a "round tree". It'll do for now – we can always change it later.

There's another option for assigning images to items. For instance, we're far too conceited to admit that we don't know what a samovar is, even though it's mentioned on the first page of Uncle Vanya, so we add it to our props list, and click on the "images" link at the bottom of the "Media" section. Cleverly, Celtx opens a browser window and searches Google images for pictures that match the word "samovar".

Oh, one of those. Right.

Be aware that the images fetched by Google will probably be copyrighted to their owners. Nobody will object to your using them as reference material, but don't be tempted to use them for anything else.

Once you've added everybody to the list, hit "OK", and your project will be uploaded to the Celtx server, including any images and other media you may have added. You can download a script in a similar way – just click on the download button, and sign in to your Celtx account. Celtx will fetch a list of all the projects you have access to. This will include any projects that people have chosen to share with you, but also any projects marked as public. Why not download a few public projects to see how other people are using the software?