Step 1: Setup Your Paypal Account, Identify Your Button ID and Embed in Your Instructable
First, if you don't have a Paypal account then follow step two of my Instructable entitled How To Link Your Instructable To A Paypal Donate Button. Once you have a Paypal account follow step four sub-step nine or go directly to the Paypal site to create a button. Once you create your button there will be some code that Paypal generates. In that code there will be a section listed as "input name="hosted_button_id" type="hidden" value="XXXXXXX" . You care about the characters in the "value" field, which I have listed as XXXXXXX. This is the Paypal identifier that links to your button/account. Copy this value and paste it in place of the XXXXXXX in the following html string
Use this link in your Instructable. You can type what you want to say, like, "If you wish to donate you can!" and then highlight that text and click the html link icon (world icon with the chain link) in your Instructable editor and assign the Paypal html link info to the URL field. Click OK and that's it!