I am not going to write your entry for you, but I will answer your question and describe the life of a migrant farm worker. First of all, you wake up. You wake up in a large common dormitory or outside. You do not have a proper bed, but you may have a cot with a thin blanket if you are lucky. More than likely the cot has bedbugs or fleas. You get up before dawn, because the crops wait for no man.
When you get up, there is no indoor plumbing in your cabin and no bathroom to visit if you are outdoors. You hurriedly wash up at a spigot or in a stream. You might eat canned goods, leftover fruit, or if you are lucky food is provided for you and you might make a meager meal of biscuits and gravy. You eat quickly so no one will steal your food and because you need to get into the fields or orchards.
Finally, the work day begins. At first it’s not so bad, unless you are sore from yesterday. Then, the sun comes up and you are roasted alive. You work long after you are tired, and you only stop briefly for a pathetic meal. You need to meet your quota, but you need to be extra careful to do it right. It’s not easy! If you are lucky, your overseer only swears at you and tells you to hurry up. It could be much worse.
After the sun goes down, you might waste what little money you earned on gambling or possibly at the company store. Then, you get to fall into your cold hard bed with the bedbugs and fleas. In the morning, you get to get up and do it all over again until the crop is in and you have to move on to the next one. If you’re lucky.
One way to approach this assignment is to do some research on the Great Depression and the migrant workers. However, it is worthy of note to know that the group about whom John Steinbeck wrote his Of Mice and Men is the itinerant white male workers who were displaced so greatly during the Depression.
You may also wish to learn about Woody Guthrie, the itinerant folk singer and voice of the disenfranchised. As Steinbeck himself wrote of Guthrie,
... Harsh voiced and nasal, his guitar hanging like a tire iron on a rusty rim, there is nothing sweet about Woody, and there is nothing sweet about the songs he sings. But there is something more important for those who still listen. There is the will of a people to endure and fight against oppression. I think we call this the American spirit.
The men of Steinbeck's novella exhibit this will to endure. So, in a diary entry, you can write about the men's having ridden on the trains and buses to find jobs, just as Woody Guthrie did. These "bindle stiffs" are mistrustful of others since they all are competing for a limited number of jobs. Fights and such were not uncommon on the boxcars as men traveled across the country. Loneliness is also a companion of these men who work harvesting hay and wheat and other grains.
As you brainstorm for ideas to write in the diary, consider what the men do on a typical day, their fatigue from working, their anxiety that a job is nearly finished and they must search for another, and their mistrust of others. Think about the interplay of the characters in the bunkhouse in Steinbeck's narrative and all the emotions exhibited in it: antagonism, despair, boredom, loneliness. What little hope they have is in the dream of owning something of their own and in having some fun in town. And, yet they endure.
(see the links below for more ideas)