How can I start on an English assignment to compose a script in which Friar Lawrence reflects upon his actions and his part in the tragedy of the...

Perhaps, the scene could open at the end of Act V as the mourners depart and Friar Lawrence remains behind, reflecting upon his own words and deeply regretting his actions. That is, a soliloquy could be created as the major part of the script.

Taking from his monologue these lines--

She wakes, and I entreated her come forth,
And bear this work of Heaven with patience.
And then a noise did scare me from the tomb,
And she too desperate would not go with me,
But, as it seems, did violence on herself. (5.3.275-279)

--the student can begin the soliloquy. The key words here are "this work of Heaven," which means, of course, fate. For, throughout the play, Romeo and Juliet have tried to defy fate (Romeo even declares, "Then I defy you, stars!" (5.1.24). The friar, too, has essayed to manipulate circumstances so that Romeo and Juliet can be married and happy while he works to reconcile the feuding families because their beloved children have acted in love, thus setting the example.
But, of course, the friar has committed the cardinal sin of pride in thinking that he can ameliorate the antagonism between the Capulets and the Montagues. Moreover, he has broken one of the vows that he has taken as a priest: the vow of humility.

Therefore, as he stands alone, reflecting upon his sin of pride as he has thought himself god-like in that he could manipulate circumstances and relationships, the priest must berate himself for his sins against Heaven and the breaking of his priestly vows. Indeed, his acts of pride have wrought the deaths of two of God's creatures. In this soliloquy, too, he may cry out to Heaven, asking why he could not have died instead of the innocent Juliet.

Perhaps, the student can begin in this manner,

Death, dismay, grief, loss have I alone caused in my pride!
How have I thought myself so designing that I could deceive man and Heaven both?
I once asked Romeo if he were not "uproused by some distemperature" (see 2.3)
When it was I who wast swelled with arrogance
Oh! shame and sin must I now bear forever.....

[Here Friar Laurence can remember certain things that have happened and think what he should have done, instead. The end of his speech can be his exclamation that he has betrayed, rather than helped the families. He has been a Judas.]