Here are some thoughts. It is always difficult to write a letter for yourself that someone else will sign.
1. Start by telling how long and in what capacity you have known the applicant.
2. Cite 2 or 3 qualities that are particularly valued with this employee.
3. Use specific examples with details to illustrate these qualities (how much the employee was responsible vs. simply following directions, if he/she supervised others, were there budget concerns/deadlines, leadership, etc.)
4. Avoid superlatives.
5. Summarize the qualities in a final paragraph with a bit more phrasing that simply listing for ex. leadership, motivation, independence.
6. End with a final phrase such as "I give this person my strongest recommendation."
I'm sure others will have more suggestions. Hope these help.
I once broke my superlative rule with an intern we had who was superlative in every respect. I started the letter saying that the reader would probably not believe all the superlatives in the letter but that I had no other way of describing the kind of job the intern had done.