I plan to start my MBA program and I was required to submit the recommendation letter from my employer. I asked him to do it, but it turned out that no one employees besides me needed this document and my boss doesn't know how to write it. I have a question: can I write it by myself and let my boss sign a paper?

If yes, are there any resources or guides that can help me??

[Edited by Kehaj on Feb 21, 2022]

Then take a look at what your target MBA programmes say are the key capacities and qualities of the applicants they they look for. Write a list and tr to cluster the most similar term together.

Try a google search with these words:
sample MBA recommendation letter

Look through these samples, and estimate the average word count. Allocate the word count into sections, aligned to the desired capacities prominence in the schools' documentation.  Think of examples from your own experience that can illustrate these qualities. Review the samples for for phrases that reflect those capabilities, and reword them adding in your experiences as the manager has observed them. Run it through Grammarly. 

Then take a look at what your target MBA programmes say are the key capacities and qualities of the applicants they they look for. Write a list and tr to cluster the most similar term together.

Try a google search with these words:
sample MBA recommendation letter

Look through these samples, and estimate the average word count. Allocate the word count into sections, aligned to the desired capacities prominence in the schools' documentation.  Think of examples from your own experience that can illustrate these qualities. Review the samples for for phrases that reflect those capabilities, and reword them adding in your experiences as the manager has observed them. Run it through Grammarly. 

Google is your friend. Try Google Images too.