This article assumes you know how to use some of the basics of blender 2.8 & how edit mode and polybuild works:
Start by making your model, next go into edit mode.
If your model is rather simple and small in terms of geometry simply go into UV Editing [which is one of the pages found at the top hot bar]:
You’ll find your page has split into edit mode and the UV editor, make sure that you press the UV sync selection box which is the at the top left of the UV editor.
If it is simple all you need to do is Smart UV project by click “a” to select all your faces, click “u” and it’ll bring up some options click “smart UV project” make sure Island Margin is at 0.03 as recommended by a tutorial I watched a while back.
You can now go to the top bar of the UV editor, click the UV box and the bottom option will be Export UV layout, then choose .png, When exporting make sure that the dimensions of the texture are power’s of 2 so (512512 or 10241024 or 20482048), this prevents stretching and corruption of the texture, then you can start painting over it in either MS paint, GIMP or any other photo-editing software of your choice.
Next upload your blender model to Roblox as a .FBX file by exporting it. You can then insert it into Roblox by going onto RBX studio and making an object called “mesh”, insert your .FBX file under properties. Once you have your texture complete go to roblox.com and find the place to upload your decal. Upload your texture png as a decal.
Next find the texture ID by going to the URL of the Roblox website the decal is found under and copy and past into the mesh’s properties which should be found under “texture ID”.
Your texture should then be shown on your model, congrats!
If your model is more complex mark [by selecting edges and the pressing “u” and marking seams] this helps blender to cut your model in order to flatten it making it more logical than blender’s auto system.
When your happy that it is mapped the way you want it press “u” and “unwrap”.
Practice is key with these things and you’ll over time find ways of getting more efficient at it
Then follow regular procedures as mentioned above…
I hope this helps everyone out there who was as clueless as me. 
Any questions just drop down to comments section.
