meaning of 把自己 and 收下 and 在地里 and some translations
收 means (among other things) to harvest
完 is used to indicate that the action is completed, finished. So
收完 means to finish the harvest, or as @倪阔乐 put it, to complete the harvest

Similarly 说完 means to finish what you want to say. 说完了没有？ "Is that all you have to say/Are you finished (talking)?". Grammar books usually discuss this 完 verb complement, so try to use them. 

把 is another particle you have to look into. If used, the sentence order is changed. The object of action follows immediately after this particle, thus it gets more emphasis. Consider e.g.

我在吃午饭。 I am having lunch. 
我把午饭已经吃完了。 I have finished eating my lunch. (Just to use the 完 again in combination with 把). Notice that 午饭 is pushed to the left of the sentence. This is an important particle and sentence structure that you have to understand. Again, consult your textbook or grammar reference for more. 

As for 收下, it simply means to receive or to accept. E.g. 收下名片 to receive a business card or 请收下这个... Please accept this..., 收下礼物 to receive gifts/presents.   

Here's one more example sentence that uses 把 and 收下:

请把它作为一件礼物收下。 Please accept it as a gift. 

把 is immediatelly followed by the object 它 (it)
一件礼物 a gift (one piece of gift, I hope you know what a classifier is)
作为 as ...
收下 accept