Less is more
As well as in numerous other situations, here the "Less is more" principle is also appropriate. Although it may seem to you that it is easy to reduce one's life to a single page, because you knew the deceased so well, as soon as you start writing, you will see how demanding it is to make a list of the most important features, works, events etc. However, besides the sincerity, this is in fact the fundamental characteristic of a memorable and solemn farewell speech.
You must not exaggerate in expressing your emotions, and you must pay particular attention not to put yourself to the forefront by what you say and by the way in which you say it. All the attention of the farewell speech is to be focused on the one who is not with us anymore, and instead it should provide consolation to the family members as well. Therefore, after having written a necrology, read it to several persons and ask them to express their objective opinion. It is difficult to write when the emotions are intense. Therefore, a second opinion and potential corrections by an impartial listener will give the necessary level of solemnity to your work.
Besides, this is an appropriate way for you to try to visualise the way in which your words at the funeral will come to life, in which places you should make a pause, which details are to be emphasised, and which still you will find very personal and thus emotional. If upon the reading of your speech you decide that you do not want to hold a speech, with all due respect to the deceased and the mourners, since you could break down, it is important to be aware of that in order not to put yourself in an embarassing situation at the ceremony.In this case, it would be better for you to let someone else read the farewell speech.