I assume you've download the media creation tool so click create installation media and then there should be an download ISO option
Once that has downloaded, right click on the .iso file and select mount. It should open in a new window
Navigate to sources\sxs and copy the address in the top bar
Once you have that address, using windows search, search for features and select the one which is called Turn Windows features on or off
Check the tick box next to .Net Framework 3.5 then click OK
Follow the instruction within there but at one point it should mention source
Select you have your own or something along those line
Paste in you address from earlier
Follow the next steps in the installer
