gorby
Now, that's a BIG question.
First you'll need a board of some kind. Some people use wood and some others use a picture frame (usually loads in charity shops at good prices). I usually use insulation board. The benefit of using insulation board is that it's easily carved and usually available free in a skip near you. 🙂
Then I often cover it in soil. You can buy diorama soil or alternately, real soil can be used as long as it's heat treated so to kill micro organisms. Mixed with diluted PVA glue it can be applied as a (very messy) paste to the surface which dries rock hard.
If you intend to use static grass it looks a lot more realistic if you use two or three different colours and lengths.
There are loads of items which can be bought or scratch-built to make dioramas more realistic, it all depends on how much work or money you are planning to devote to the project. More detailed advice depends a lot on the type of dio you are intending.

John Hughes
Alternately, there are premade dios which require minimal assembly and painting. They're a good option for a beginner.

Steven Van Dyck
Look for a picture of the real thing and then imagine how you could cut out a piece of diorama from that. Keep it small enough so the focus will stay on your model.

jaero Author
thank you guys for the help! i reckon i could go the foam dirt and oven approach, i will update this post as i go through making this diorama!

Kesa Tiho
As john said you can buy pre made dios, italeri makes great dios, accessories and minifigs for dios. Also a company called noch makes synthetic foliage and pre built trees, dios, figures, etc but the scale isnt listed. There are many more companies like tamiya that make stuff for dios but i dont want to go on a rant, rather id say you should trust what gorby says because hes been in model building for maybe twice my entire lifetime

jaero Author
i see, i might try italeri parts but use the advice from great sage gorby to sculpt the basis. thanks for all the tips and info!

gorby
Who's this Gorby sage? 😄 😄 😄
Sage is more likely to refer to my colour than my intelligence. 😄
I keep meaning to post my experiments with using real soil on Scalemates. If I get a chance I'll do it today.
And Kesa, I've been modelling since 2015. I did liberally pour glue onto various plastic shapes when I was kid, but I can honestly say that I don't think I learned anything useful. 🙂

Treehugger
What comes to mind, me not being familiar with making dioramas: Tank tracks tend to reflect whether they are used on road, or off road, as the on-road tracks tend to have rubber parts on them and probably aren't used off road as they might lack the tracksion of an all-metal-track setup.

gorby
Jaero, this is my album about using real soil for dios.
Using real soil for dioramas | Album by gorby

jaero Author
thank you! i will check out this album now, and thanks for the track mark advice treehugger!

grappa_0_0_0
I recommend following a guide on youtube. Night Shift makes the best dioramas you could dream of, but there are many other good channels. Also, I recommend being cautious about DIY solutions, as well as biodegradable materials. If people who make lots of dioramas and bases use certain pre-made products, there's usually a reason for that. In other words, don't ruin your projects just to save a couple bucks on materials. Naturally, it also depends on the level of realism you want to achieve.

grappa_0_0_0
Another thing: what works in dioramas and what doesn't isn't really obvious. If you start from scratch using only your imagination, you may nd up with a very unsatisfactory result, since just because something is realistic it doesn't mean that it's also perceived as pleasing to the eye of an observer. It's hard to predict what our brain will find harmonious. However, experts know the do's and don't's of diorama making by heart, so it's important to use their work as a base for your projects, which you will modify to fit your needs and imagination. There are magazines and youtube videos for this sort of stuff.

Treehugger
I am guessing that dioramas are always best with relying on some reference photos to really nail down details and make things look natural.

gorby
I personally think the main reason many people use pre-made products is because it's easy and they don't know it's possible to make many of the products you can buy. It also takes more time making your own. There is nothing wrong with using pre-made products just as there is nothing wrong with using natural products as long as they are prepared correctly.
These are two of my dioramas in which the only bought product is static grass:
100% Scratch-Built Angry Slug – A7V in 1/48! | Album by gorby (1:48)
The next one is the dio I spend more time over and therefore is the one which turned out the best in my opinion:
Dealing with my hanger issues. 1/48 Scratch built Hanger dio. | Album by gorby (1:48)

jaero Author
thank you for these tips grappa! i have been wracking my head with night shift for a bit to try figure out foliage, and i will do my best to not skimp on materials! this angry slug is beautiful, i think making what i can from expert teachings and buying some parts might be my key