Most new take-off doors will probably have the hinges still attached--that's good, saves painting a new set etc.
Mounting the shells is pretty straight forward and can be made easier if we know whether the new doors have hinges. Two hex bolts for each hinge where they attach to the body or the door shell, each with a relatively large amount of movement or adjustment to fit the body properly. The wind catch is held in place with Torx drive screws. Save everything you removed because its just a good practice.
With or without hinges already attached doesn't affect the process of properly fitting the doors to the body----for the most part its a trial and error thing. I'd begin by loosening the bottom strike (don't remove the countersunk Torx drive bolts all the way---they hold a threaded plate inside the pinch weld area) and move it towards the center of the van. Remove completely the top strike.
The front-most door might have electrical connections running from the B-Pillar inside the door----they need to be removed and disconnected from inside the B-Pillar. At the minimum you'll have the door pin switch for dome light operation---you'll also remove that wiring harness along with any other harness or connector running inside that door. Once the connection inside the B-Pillar is loose the large rubber boot can be pulled loose from the body, wiring harness and connector coming along too.
Using a removable marker trace around each hinge (door or body) as a place to initially mount new doors. Tighten bolts just snug enough to allow operation but not fully as you may need to move the hinges small amounts before achieving the best fit. Because the doors won't engage the latches you're pretty much free to adjust this rearward door until all three edges closest to the body and the gaps are "perfect". Once you're satisfied with the fit re-install the strikes. Top is fixed in place so nothing special up there. The bottom does have adjustment capability---it will be properly adjusted when its latched the door shell vertically aligns with the body and slightly compresses the door seal.
You want the handle operation to require minimum effort, don't want any sort of hard pulling required or a "spring" outward as the latches release, just a firm hold on the door. Once everything is adjusted make sure the upper and lower latches are fully engaged and don't let the door move in and out when pulling against its edge closest to the center of the door opening.
Smaller door doesn't require as much adjusting etc although it does have to align with the upper and lower body, the rearward door AND the front door too.
Once everything is adjusted and working right tighten the hinge bolts, install the wind catches (grease the slides on these) and remove the double layer of tape. You should now have side door factory windows.
As you get closer and maybe need more info post again and we'll try to add more.
HTH