How do I make an answer as the best answer?

You have to wait 1 hour after you posted the question. Then you can pick one.

You can't until the question has been posted for at least an hour. When you click on your question after the hour has passed, you'll see an invitation to pick a Best Answer. Just do what it says.

But I do recommend not being in a hurry to pick BA. You never know who else might post an answer in the next few hours. There's no harm in waiting a full 24 hours to allow all the users from all around the world to have a chance at answering.

as long as the question has been up for at least 1 hour go to the question under the answer you want to pick click on choose as best answer give it a rating then click submit

Wait an hr check your Yahoo and select a best answer with stars

just click on their answer as best answer

Choose and rate a Best Answer in Yahoo Answers 

Awarding a Best Answer in Yahoo Answers lets the community know it was helpful. You'll have the option to choose a Best Answer 1 hour after asking your question. 

Sign in to Yahoo Answers. 

Click your username in the right column. 

Click a question to open it. 

Click Award Best Answer. 

Select a rating. 

Click Submit.

Once your question has been posted for at least 1 hour and has at least one answer, click on 'Award Best Answer' button next to your chosen answer.