Since there is no one who can vouch for your employment but yourself, you need to write something similar to a CV. Also be prepared to add docs for this, too:
1) Start with your training for your profession. You might add diplomas, admission-papers, etc. to prove this.
2) Write about your profession itself (with supporting documentation, i.e. tax-returns, bills, etc.)
3) Write - in detail - about what you do. This must correspond with the NOC you apply under. Do not make the mistake of copying the NOC-description verbatim, because this will create suspicion on the part of the Visa Officer. Rather look at the NOC-description and then write a bullet-point-list in your own words about what you do. This should be at least similar to what tne NOC requires.
To verify this, you might want to add documents, i.e. letters of people hiring you for your service.
The really tricky part is to prove that you have been doing this for the number of years you claim. There is no help with this: you might want to supply hiring-letters from clients for all these years (maybe one per year, maybe 10 or more). You will never really be sure if it was enough until you get a decision from the CIO.