Simple:
* Sam-yew-el Mc-Klen-don
* Sam-u-el Mc-Klen-den
More Detailed:
* Sam-yoo-el Mc-Klen-don
* Sam-yoo-el Mc-Klen-dən
Very Detailed:
* Saym-yoo-el Mc-Klen-dən
Ultimately, the best phonetic spelling depends on the specific pronunciation you are trying to capture. You can choose the spelling that best reflects the way you or someone else pronounces the name.