Process writing, as the name suggests, focuses on writing as a process and not as a final product. It involves helping students see that a piece of writing goes through a series of revisions and editions. What is put onto paper initially does not need to be polished or perfect. Students can begin to see that a piece of written work doesn't need to be all correct from the first try at it.
When planning a lesson on process writing, it can help if you think about the stages that you go through when writing. My suggestions are: brainstorming and noting any ideas that come to mind, deciding from these ideas which are most relevant to the question or task, deciding how the piece of work will be organised (this will probably be in the form of a plan). These are all important stages to develop and practise before writing starts.
Using the plan, students can start writing. The focus for the first draft will be on content without giving too much thought to grammar and accuracy. This is not to say that grammar is not important, just that it is not the focus at this stage. Students then go back and revise the first draft a number of times, initially looking at content, relevance and organisation of ideas. When they're satisfied with the content, they can move onto looking at grammar, vocabulary, spelling, punctuation and connectors (however, moreover, etc). Hopefully, some of the errors will have been detected whilst looking at the content.
This can be a novel way for students to approach the task of writing. It can be very liberating and can produce lengthier pieces of work. If you have access to computers, you can do some of the activities there. Computers lend themselves very well to this sort of activity.
I hope you and your students enjoy these activities. I have always found that students appreciate this guidance and generally produce better pieces of work as a result.