"Do one thing" is a phrase that is commonly used to instruct someone to focus on one particular task or action at a time. However, there are various synonyms for this phrase that can be used to convey the same message. Some suitable alternatives include "concentrate," "focus," "prioritize," "single out," "zero in on," "zero in," "home in," "undertake," "devote oneself," and "determine." These words can be used interchangeably to effectively communicate the importance of completing one task before moving on to the next. By using synonyms, you can add depth and variety to your language, making your instructions more effective and engaging.