First of all, you have to download the SDKBETA version, if you already have it then skip this part.
Right click your game, last option, betas tab, select SDKBETA, let it download.

Now that you have the SDKBETA, open your game and you'll notice there's 2 new options on your menu, "workshop" and "installed maps", go to workshop and find a map that you like, since I wanna farm some achievements and xp easily I'll be using a map called "XP GRINDER 1.2". After finding yourself a map, go to the installed maps and check if your map is in there.

NOTE: Dont take more than 1 map, I'll explain later.

Create a solo game and on the map list, select your new map and configure your game to a difculty you can at least finish the game, since the map I'm using is for xp grinding and achievement farming all dinos will spawn on a unreachable place while all I have to do is shoot them, so I can use the highest configs to get a huge amount of xp like prehistoric with suicidal configurations (dinos with 400% damage and life, no credits, no vehicles, etc).

NOTE: Be careful choosing your configs, if you wanna play workshop maps to get achievements and xp, you won't be able to change it later, I'll explain it later too.

After configuring your map, play it to check if the configs you set are good for you, if you see something's wrong get back to the map configs and change it again until you finish the game or until you die at a high wave.
Once you're sure that this map will help you and the configs will boost the rewards, don't touch anything more in the map configs and just close your game, Orion has a feature to save the last configs you did to your map after you finish playing it.

Finally, right click your game, last option, betas tab, change SDKBETA back to the empty option and let it download again, I know that the worst part of this guide is that you have to basicaly redownload the game, but it's the only way I've found for it to work.
When it's done, open your game, you'll notice that the menu won't have the "workshop" and "installed maps" options anymore ;-;... BUT, if you create a solo game, you'll find it already configured to run the map you installed with the same settings you did in the SDKBETA version :D Just click on play and have fun.

Q: Why I can't install more than 1 map?
A: Because when you get back to the original game version, you won't be able to select a map from the workshop, and the feature that saves settings from the last game will aways be set to the last map you played. So theres no reason for you to have 100 maps from the workshop if the game will only save 1 map.

Q: Why I cant change settings after returning to the original game version?
A: Because everything in this guide can be considered a glitch, so if you make any change on the map settings such as changing diffculty or starting credits, the map will be automatcaly changed to one of the original maps of the game, and you won't be able to select your map anymore. That's why you have to configure your game to your liking while you have SDKBETA version installed.

Q: I changed a setting on my game and now the map is no longer selected, what now?
A: That depends of what you have done so far after realizing that the map was no longer selected.
If you have made a setting change and noticed that your map is gone before starting the game then all you need to do is restart your game, after that your map should be selected again, don't touch anything else and just play the game.
Now in case you have started playing the game without having your map selected, you'll have to do ALL THE STEPS AGAIN FROM THE START, SO DONT LAUNCH A SOLO GAME IF YOU SEE THAT YOUR MAP IS NOT SELECTED!!!

Q: Are you sure this guide works?
A: Honestly, I made it a minute later after noticing that it actually worked, but I'm not sure if it will works for everyone. Plus this guide is already pretty old and I haven't played it in ages, but you can check the latest comments to see if anyone's confirming that it still works.

Q: Does it works for online play?
A: I don't know because I never tried. Maybe it's possible and maybe it's not.

Thx for reading.
I really hope this method works fine with you guys too, some people says it's possible to get achievements while playing on SDKBETA but that didn't worked for me so I've found this workaround and thought it could be useful for everyone.