A cover letter has one job: to connect what the employer needs with what you have already done. Two relevant examples and a clear reason for wanting this role are worth more than a page of enthusiasm.
The role and where you saw it.
Name the position and the organization so the letter is clearly written for them.
Two relevant achievements.
Choose experience that matches the posting, and say what the result was.
Why this employer.
One specific reason: their product, their mission, a project you admire.
A confident close.
Say you would welcome a conversation and how to reach you.