When building dioramas I like to use a piece of paper & draw out exactly where my wall studs, headers, etc. are going to be. Remember these wall studs are going to sit inside the hardboard walls so make sure you have your figures right so they end up where you want them.
Side walls or supporting walls are usually laid out 16” on center, some old buildings are 24” on center & some old non-supporting walls are 24” on center. But the rule of thumb is 16” on center for all walls supporting or non-supporting. If you are replicating an old building check it out so you can make it just like the original. Codes & rules were a lot different in the old days.
Figure out were your top plate & sill plate will be located & draw these lines on the paper. Now figure the proper length for your side studded wall & draw the end lines. Remember the ends of the side studded walls are going to be made with triple 2”x4”s glued together so you have somewhere to glue the end studded walls to. Now draw a line right in the middle & then figure 16” on center either way, this will give you a nice layout & it will give an even looking wall. When you draw the line make them go past the sill & top plates so you can see them when it comes time to assemble the studs.
You can use clear tape to hold the top plate & sill plate to the paper, attach them to the paper. Glue the triple 2”x4” ends together & then glue them to the top plate & sill plate you can use tape to also hold these in place if needed until they dry. Once that has set up start assembling the wall studs using the lines you drew on the paper for placement, now is when you will appreciate that you drew the lines past the top & sill plates so you can line up the wall studs perfectly.
Now do the back end wall the same as you just did the side wall, except this wall will only have single 2”x4”s on the ends not the triples (the triples are only used on the ends of the side walls). Next do the other side wall this is the side wall with the walk in door. Once you have assembled the top & sill plates and the wall stud ends start working on the doorway. The doorway will have a 2”x6” header above the door then little short 2”x4”s above the door header. After the doorway is complete finish this wall with the 2”x4” wall studs.
Next work on the front end wall with the big door; frame out the sides, top & bottom just like the other end wall. Now install your header above the doorway, in the building I built there was very little room for 2”x4”s above the header so I just installed an extra 2”x6” which actually made it look better & this is how they would have done it on a real building if in the same situation.