I have been my own boss for over four years.
Question1:
How do I write referrence letter for myself?

Answer1:
If possible, you may ask your partner to write a letter of reference for you.

Question2:
My previous employer before I started on my own is nowhere to be found. How do I write letter of referrence about my previous working experience?

Answer2:
If the company is still in business, you may contact the Personnel Department and ask for an official reference of employment.