First, quite obviously, you will need to place a star for objects to revolve around. I recommend using either a very low end M-class star (the smallest and reddest stars) or a brown dwarf (a very, very large gas giant, preferably above 70 Jupiters in mass) You can, technically, start out with other sized stars but I recommend that if you insist on doing so, turn realistic off, and the luminosity to zero. The reason I say this is so that when you're placing your planets to form, they will actually collide and get larger instead of heating up and disappering before ever hitting anything else. Also, another reason I don't tend to use larger stars is that the system in general is much larger, and so the likelihood of a collision goes down, therefore making you wait longer for things to become more stable and more like a fully-formed star system.

So, in conclusion to this section, I recommend using a Brown Dwarf of Red Dwarf star, and if you don't, remember to turn the luminosity of the star to zero.

Now that you've added your star, it's time to place what will become your planets. The way I typically do this, that works quite well, is to use the feature "random moon" and place those however I want. I typically use them in a full disk pattern if I want, but sometimes I do it in a sectional pattern (see picture) Though I normally place a few tens of these, maybe 3 or 4 hundred tops, total. Though if your system can handle it easily, then go ahead and place some "random small moon"s around too. The fewer objects you have, the better performance you will get, but also less collisions and smaller planets. Though your density and placement of these "moons" or planetesimals give a large impact on what types of planets will exist when your sysem is finished, which I will discuss in the next section.

The placement of "random moon"s will give your planets their mass as they collide and become larger. The denser you pack these planetesimals, the greater the mass of the planet in that region will be.I typically have the most tightly packed part of the system nearest to the center, as many of these objects will be knocked out of the system or into the star/brown dwarf in a later step.Also, try to keep the number of "moons" at around 30 whrever you want an earth-sized planet, as I've noticed that most of these "moons" are about 2 moons in mass, give or take some tenths. In the picture below I have shown where I normally "put" my earthlike planets around brown dwarves (Only, in other types I put them in the habitable zone of the star)

But why there? Why would I put them in some arbitrary spot rather than in the habitable zone of this brown dwarf? That is because Universe Sandbox currently does not calculate habitable zones for brown dwarves, so I approximate where it might be if it did. (Note: in the picture, I only show the placement of inner solar system bodies, as I don't normally place those outer ones for the same reason I don't use large stars)

Now that you've placed your planetesimals, through random clicking, you probably placed some objects above or below the orbital plane of the system. If not, great, you were careful! If so, then to flatten out the system, try using the "Make 2D-- zero all Z values" option under powers>more. Though, if you're willing to wait much longer for the system to become stable, you can leave them as-is, as that will give your ending planets some inclination (none of such is a bit unrealistic) though, it's much easier and faster to just add in the inclination once the system has become fully-formed and stable.

Now, until this point, you've hopefully been using auto-orbit option to place the "moons". Well...That presents a bit of a problem. All the orbits are perfectly circular (see picture) and isn't realistic, and won't get you very far on having things collide and get bigger.The way I normally fix this is to play the simulation for a bit (Hopefully it's been paused this whole time, as having it unpaused makes things harder and gives different resuts) and as it plays, constantly be playing with the "+2% velocities" and "-2% velocities" buttons under powers>more. Though, when doing this, try to undo every action, meaning for every "+2% velocities" you make, press the "-2% velocities" a similar number of times, (it doesn't have to be exact, but just try to get it close) though wait a few seconds (15 to 20 should be best depending on the stellar system size, for a large one maybe more would be better) this should make orbits more eliptical (less circular) making some orbits cross (especially if you do the back and forth increase/decrease velocity thing for a long time) and therefore more collisions occur, and planets to form larger and more quickly. This has a huge effect on the most inner part of the disk but the farther out parts won't be effected as easily, just a warning. after most of the colliding that can be done, has been. you may want to set some of your larger planets back into circular orbits, leaving the smaller ones in crazy orbits, by going to the large planets' properties, then going into "actions" and clicking on "auto orbit". if you want the planet to have an eliptical orbit but just not a hugely eliptical one, you could either not hit auto orbit and change the eccentricity of the planet's orbit lower, or do set it on auto orbit, then change the eccentrity as high as desired for the planet (or as high as you want and still maintain being in orbit, and the orbit being realistic)

All that is needed of you now is time, keep running the simulation for as long as you want or can, and the longer it runs, the more stable your system should become, and the closer to a fully formed one. Also, once you're satisfied with the stability of the system, you can edit the planets not too far past their orginal parameters but also to your liking, except the temperature. By now, all of your planets probably are above 4000 degrees Celsius---unrealistic even for some of the closest planets to your star especially if it's small or a brown dwarf. And also, I have noticed that the planets don't cool very quickly anyway, so it's not easy to just wait, so what I normally do is set the temperature to about 0 or 1 degrees and let it change from there. At this point, if you started with a larger star than what I use, you can press "realistic" and the luminosity of the star will return to normal, and so will it's temperature. It's possibe that if you used a very large star (especially very blue stars, bluish white or white are bad about it but arent super bad, and yellow and orange stars are okay, just a bit too big for my taste) then some of your innermost planets will get so heated that they disappear, or are, in essence, vaporized. Also, wait awhile until the planets get to their native temperatures, then go ahead and terraform the ones you want to if any, and if you do this whole process like I do, you can add your gas giants now, along with, if you want, asteroids, "comets" and dwarf planets, and even actual moons. Before adding moons though, I suggest putting inclinations to what you want, because if you do that after placing moons you will have to delete and replace them because moons don't move wiith planetary parents...

Now your system should be fully formed, and savable.

Thank you for reading this guide, and I hope it helped everyone :-)