Visit this website: https://www.simplerockets.com/Mods

Then find a part mod you like and click on it.

https://steamuserimages-a.akamaihd.net/ugc/2059886408435041056/28DDBF15207EE6E7B25B7A6BA23D7FB2BCF67EB4/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

This will open

https://steamuserimages-a.akamaihd.net/ugc/2059886408435043412/7700EACB10294A7B4CFCCACDA988C2240895D362/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Then, scroll down and read the mods description, and stats and comments if you want.

Under the stats there should be a download button that says PC/Mac or something like that.

Click it and put the mod in a directory you want (like downloads or documents)

After, click your windows button and type in "%appdata%" then click on appdata which is before roaming, and then you will see 3 folders, open "LocalLow" and search for Jundroo, after that you can open the folder that says SimpleRockets2 or SimplePlanes (the tutorial works for both games)
and click on the "Mods" folder and put the mod file we download earlier in there.

When done, close everything and open the game going into the sandbox company of your creation,
in the top right it will say "Modifications", This is normally where the career button is in Career, once opened it will show a list of installed mods, simply enable it and go into the game, have fun.