First off, i want to preface this. Alot of this guide is based on the equations and videos of the worldbuilding genius Artifexian on Youtube. Hes great. Check him out! I dont claim to have come up with the equations, this guide is basically just a text version of his videos.

EDIT: Artifexian recently gave a nod to this guide in his podcast! :)

2022 Edit: Artifexian's Youtube video series "Artifexia" has basically made this guide irrelevant (that's a good thing!) - If you want an up to date realistic guide to solar system worldbuilding, please refer to that series instead.
https://www.youtube.com/watch?v=E-9U0wJxGBA&list=PLduA6tsl3gyiX9fFJHi9qqq4RWx-dIcxO

So, lets get into this. Lets start off with your home star. Your home star is the parent of pretty much everything in your stellar system. Everything connects back to the home star. When we're choosing a star, we should first choose one of the spectral types. We have O, B, A, F, G, K, M. These are classifications of the mass of a star. O is most massive, M is least massive. Our sun, Sol, is a G star. O, B, and A stars die too quickly, that doesnt give us time for exotic life to evolve. F, G, K, and M are better, but M has such a low mass (and therefore low heat) that your planets would need to be so close that all the stellar storms would just ruin your system. Large F stars put off too much UV light which destroys atmospheres. From G and K, we can have a minimum mass of 0.6 solar masses and go up to a maximum of 1.4 solar masses. Lets go with 1.26, the same as Polaris Ab (Technically Polaris Ab is an F star, but the solar mass is only 1.26, so we wont worry). So now i have my star's mass, which is 1.26 solar masses. Keep in mind that you can choose any number between 0.6 and 1.4 and the following calculations will compliment your choice.

Now lets get a few quick equations out of the way which are going to help us later. Luminosity of a star is given from M^3 (to the power) where M is the mass of our fictional star. Diameter is given from M^0.74. Surface temperature is given from M^0.505. Its lifetime is given from M^-2.5. All of these numbers are in a ratio-type form related to Sol. Basically what that means is that Sol is always the number 1. So we now know that our star's luminosity is 2.15 of Sol, the diameter is 1.19 of Sol, the surface temperature is 1.12 Sol and it's lifetime is 0.56 Sol

Lets get back to the home star fuss. Its significant, because its what is pulling everything in. But how much can it pull in? The next equation we're going to look at is what i like to call the Gravitational Planetary Boundary Zone. Thats basically my sciency way of saying the zone where stuff can stay in orbit. If a planet gets too close to the star, its going to get sucked up. If it's too far away, its not going to be able to keep a stable orbit. The inner limit equation is 0.1 * M (M is the mass of the star. It will be the mass of the star for the entire guide). The outer limit equation is 40 * M. Since our M is 1.26, our inner limit is going to be 0.126Au (Au = Astronomical Unit, the distance from Earth to Sol) and our outer limit is going to be 50.4Au.

Theres another boundary we need to make. This one is a bit simpler. Its called the frostline, and beyond this point everything becomes very COLD. The frostline should be home to your largest gas giant, which should also be the closest gas giant to the sun. The equation for the frostline is 4.85√L (L is the star's luminosity, which we calculated a while ago) so our frostline is at 7.111Au. We're going to put our large gas giant 1Au from the frostline, on the side away from the star. This places our gas giant at 8.111Au from the star.

Now before we add heaps of planets, lets setup our habitable zone. Our habitable zone equation is √L, where L is the star's luminosity again. So our habitable zone value is 1.466Au, lets call that H. But wait, we cant stop there! The habitable zone is a ZONE, so it has an inner and outer limit. The inner limit is at about 95% of H, so its at 1.39Au away from the star. The outer limit is at 137% of H, so its at 2.01Au away from the star.

Now we can add planets :) We grab the gas giant we made earlier, and we multiply the distance from the giant to the sun by a number between 1.4 and 2. Go ahead and go to random.org and randomize a number between 1.4 and 2.0, because this part doesnt use much science. It can be random. I ended up with 1.41, so the next planet from our star (going away from our star and our gas giant) is 1.41 times the distance from our gas giant to the star. This puts the planet at 11.44Au from the star. Now, we randomize again (I got 1.8) and then multiply the distance from that planet (11.44) by 1.8 and we get 20.59. We can keep randomizing and multiplying until we get our furthest planet, which can be no further than the gravity limit we set before (50.4Au). I ended up with 8.11Au (the gas giant) then 11.44Au then 20.59Au then 35Au then 48.99Au. And we can stop there, because 1.4*48.99 would be larger than 50.4.

Now we do the same thing, but backwards. We take our gas giant's distance again (8.11) and divide by a randomized number between 1.4 and 2. So we're doing the exact same thing except dividing instead of multiplying. Keep going until you reach the inner gravity limit we set before (0.126Au). I ended up with 15 planets. The distances for those are
0.128
0.179 <--- unstable orbit. needs to be removed :(
0.286
0.487
0.828
1.408 <--- habitable :)
1.971 <--- habitable :)
2.76
4.14
5.79
8.11 <--- largest gas giant
11.44
20.59
35
48.99
But now we have a problem. We cant have 2 planets within 0.15Au because of gravitational interferrence. So we'll get rid of that second one. You'll need to make your own adjustments and remove any planets that get too close to eachother. So now we have 14 planets. And now we have a stellar system. Awesome. You can stop here, but im going to get a tiny bit more complicated

In order to make your stellar system a true masterpiece, you're going to want to play with the orbital mechanics. As a foundational rule, dont move the planet and dont touch anything that isnt in the orbital elements section of the planet editing menu. Most importantly, dont touch the semi-major axis.
Eccentricity is how much your planet's orbit ISNT elliptical. Elleptical means round. An elleptical circle is a perfectly round circle. Despite common misunderstanding, orbits are not fully elleptical. The table below houses the eccentricity and inclination of the orbits of the planets in our solar system. Plus Pluto. As you can see, the eccentricity varies between 0.0068 and 0.2488. Go ahead and use random.org again to randomize and get a value between those 2 decimals. This randomizing isnt for any particular reason other than because i dont know enough about eccentricity to be accurate. Sorry, I'm not a professional at this stuff. Do the same thing for inclination (lowest 0, highest 17.142). Randomise these 2 values for all your planets and you will have orbits that look cool, and are pretty much accurate to what you would expect. Here is a table of Sol system's planets for reference.

Final product time! Here's some screenshots

Closest view
Zooming out
Zooming out more
Furthest away

I hope this helped you! Thanks for taking the time to read it. Please feel free to make suggestions or correct me.