A rational mind formulates a plan by determining the envisioned end of the plan and considering in turn the proximate means necessary to reach said end. Such a plan is executed in reverse the order of its formulation, the ends following the means. If the logical arrangement of God's thought is such that the decree to create men is logically prior to the decree that man should Fall, then the very purpose of the decree to create is hinged upon the decree that man should Fall. This, however, means the logically prior decree is arbitrary: if the meaning of any decree is determined by a logically posterior decree, then any counter-factual decree could conceivably give meaning to a logically prior decree.
For instance: is the creation of man accomplished by means of man’s Fall, or man’s Fall by means of creation? Surely the latter is the case. For the fall of mankind to be possible, it was necessary – not arbitrary – for God to create man. Parallel (in reverse proportion) to the execution of the decrees, the decrees themselves are necessary insofar as they stem from God’s righteous character. To manifest His glory, it was necessary that He decree the redemption of elect sinners in Christ and the condemnation of reprobate sinners. Likewise, to redeem elect sinners and damn condemned sinners, it was necessary that such persons exist; hence, the decree to create was necessary. While in the historical execution of God’s plan it may be that creation precedes God’s glory manifested to creation, in the formulation of God’s plan to – ultimately – manifest His glory, we know that the intent of creation is to show “that now, through the church, the manifold wisdom of God should be made known to the rulers and authorities in the heavenly realms” (Ephesians 3:10).
God didn't begin to build a house without having any logical conception of what the house would look like. If that were the case, the building would be an arbitrary construction. Rather, God had an image of the house, analyzed what pieces He needed to construct it, and decreed that such be effected. For God's glory to be fully manifested (the house, the first decree), God needed to exhibit His power, wrath, mercy, wisdom, compassion, grace, &c., so He decreed that circumstances be such that He could exhibit those attributes. Those attributes were best exhibited by the redemption of elect sinners in Christ and reprobation of the rest. Hence, Christ needed to sacrifice Himself so that He could redeem the elect sinners. But there must be sinners in order for Christ's sacrifice to mean anything, so the Fall was decreed. And, of course, men must exist for any of this to be sensible, so the decree to create was the final, analyzed piece of God's design for the house, after which He could begin construction by the historical execution of said decrees in reverse order by taking the analyzed pieces and building one upon the other.
The supra-lapsarian posits that the "logical order of God’s decrees" is, simplistically, as follows:
1. God's glory manifested.
2. The manifestation of wisdom, power, wrath, compassion, love, mercy, &c.
3. The election of some sinners to salvation in Christ and the reprobation of the rest of sinful mankind.
4. The application of the redemptive work of Christ to the elect sinners.
5. The redemption of the elect sinners by the work of Christ.
6. The fall of man.
7. The creation of the world and man.
Finally, a point that I think is under-emphasized is that while all of God’s actions stem from His desire to manifest His glory (cf. Ephesians 3:8-10, 20-21, Romans 9:23, 11:36, &c.), this decree is in no wise arbitrary because it is not predicated upon another decree. Rather, it is necessary as well, for it is predicated upon God’s holy character. As was alluded to previously, God’s righteousness itself implies that God must give Himself the most glory; that is, a different possible world, conceived counter-factually, could not more greatly manifest God’s glory. Romans 9:15 (among other passages) functions as an argument for this, for the righteousness of God is justified precisely because God’s unconditioned distribution of mercy and compassion manifest His glory, honor, and name (cf. Exodus 33:18-19). All things – including sin, without which we could not fully know God’s excellence (His holy wrath, justice, compassion, mercy, grace, power, wisdom &c.) and give Him His due praise – are decreed because God is who He is: incapable of denying His righteousness, i.e. unswervingly allegiant in upholding that which is infinitely worthy – His glory.