我觉得句中的 trying 不宜换成 to do。

因为这里的 first 不是表示“第一个”，而是表示“第一次”，它在此是副词，修饰动词，这里的 are trying 是进行时态。句意：

When you are first trying to think of ideas for an essay, put your pen to your paper and write nonstop for ten or fifteen minutes without letting your pen leave the paper.

当你第一次想写一篇文章的时候，把笔放在纸上不间断地写10到15分钟，不要让笔离开纸。

又如:When such product was first being sold, it is usually very expensive. 当这种产品刚上市时，它通常都很贵。