Just in case you couldn't figure it out yourself, or if you have nothing better to do.
Step 1: Getting Started
If you haven't already, make an account. On the homepage you will find the sign up link in the top right corner. By clicking it you will come to a page with several options. Option one says Get Started, and allows you to get a FREE account, which may be your best option at this time. If you decide that you would rather go the extra mile, opposite of the free account is the Pro Account Pricing. This shows the different prices per month and the payment types that are accepted. You can chose from $1.45/month, billed annually, $2.95/month, billed quarterly, or the best value of $29.95/2 years, a one time payment. Not sure what the big deal is with a Pro account? Fear not, because at the bottom of the page the site lists all advantages of the Pro account compared to a free one. Now, after making your difficult decision, you will come to a screen asking for your email, username, password, age(can be hidden), gender, and ZIP code.
Step 2: Ideas
Now let's assume you don't already have an idea for your instructable. Don't worry. You can look around first by clicking on the Explore button on the homepage, near the top. This lets you explore many many other projects, in areas such as Food, Living, Play, Technology, ect. Too many projects to look at? Well, you can also use other search options to see editor's picks, popular projects, recent projects, and most viewed. Still not sure what to do? That's why they have contests. Right next to the Explore option is the Contest option. Here, they have various contests for different types of projects, from gaming to school teaching, all with many possible prizes. By now you should have an idea for your own instructable, but if not, just keep searching. I'm sure you'll find one.
Step 3: Starting your Instructable(Step by Step)
When your ready to create your instructable, click the Create button in the top left. The next screen gives three options, Photos, Step by Step, and Videos. For now, clicking on the Step by Step option should bring up a page like in the photo above. Here, on the first step called intro, you may, what else, write your intro. Your intro will appear on the main page of your instructable, above your pictures and the first real step. When done with your intro, you can click either the Add Step+ option or the insert Step option to get your first step to your instructable.
Step 4: Step by Step Instructions
On the first step you should tell people how to get started on building or otherwise making the instructable you are presenting. This should include mostly the materials needed, and if possible, where to get the materials if one does not already have them. You could also explain why it is useful, or whether it is just fun to have. The next part is to add a photo to go along with your instructions. First, make sure the picture is on the device you are using to upload with. Then, at the bottom of the edit page, click Select Files, find your photo, select, then press Upload. Be sure the photo is uploaded to the correct page so you won't need to edit as much later. SHould you not complete the entire instructable in one day, simply click the Save Now button and continue work later.
Step 5: Video
Should you wish to include a video in your instructable, above the body text there are several options, including Bold, Italic, Link and Unlink, Remove Format, and Embed Video. By clicking the embed video option, you may paste the embed code from another site, such as youtube. To do so, go to the site and click the Embed option, which should give you the code to use. Then copy this code and paste it when prompted on the instructables embed video option and press OK. This should embed your video, which will appear when viewing your instructable.
Step 6: When done
When you're done with the writing of instructions and uploading of pictures, go back and review your own work. If you feel like a step should be before or after another step, click the Reorder Steps button. This lets you place your steps in the order you need them to be in. If you feel like a step has little use or just takes up space, click the Delete Step button. Make sure you are viewing the step you wish to delete, click the button, and agree to delete the step.
Step 7: Choosing Category, Entering Contest, and Publishing
When you feel like you have done everything you could and are ready to publish, click the Publish button near the top. This should bring you to a screen that lets you chose the category to put your instructable in, lets you enter a contest with your instructable, and lets you chose which license you wish to put on your instructable. You may now choose which Category in which to put it, including Food, Living, Outside, Play and Technology. Under each category are tons of subcategories to chose from, giving you the option to make your instructable in a specific area or one that ranges. It also offers a custom category for you, should your instructable not fit under the others. Then you decide not only how much it could cost, but how difficult it is to accomplish. You may also enter your instructable in one of various contests. Once done with all of this, you will be asked to choose which License to put on your instructable, which affects how third parties treat your instructable. You can choose to let the third parties use it, build upon, change and otherwise redistribute your instructible. Or you can keep it to yourself, although Instructables.com automatically has permission to republish it on this site and partner sites once it is published.
Step 8: The End
For now, I have imparted most if not all of my knowledge on how to make an instructable. Should this instructable not be informative enough to help you out, the site offers extra information and tips for creating one yourself. JUst click the Create option at the top left, then click the Learn more & Get tips option underneath each instructable creation type. If I have missed any features anyone else knows of or has come across and thinks they may be important, please tell me so that I may update this and make it more informative.