Download editor 2013 from steam (its free of charge) then open this up.
(I am sorry if i have missed a few steps below, but i cannot remember every single thing about editor-but you should have no problem following this basic step guide
Once screen is loaded, load database. Try to find the team under 'clubs' using 'contain' (if found-ignore next 2 paragraphs). If not you have to 'create' the club.
Select English league rules and go to 'club', 'stadium' etc to create new club/stadiumand whatever else you want as fictional.
Create new club (for example) and play around with 'kits', 'history' etc (or leave as default). Theres a lot to do here/or not alot, dependant on your choice (such as either adding staff, ground etc).
Add players and staff (or leave it to generate players).
Select English leagues and go to 'competition'.
Go to 'nation' then 'contain' and type in 'english'. You should find the desired league.
Double click (i think) the 'add team'. Do not add team for next season, your team wont be in the game?
Save etc and load database when starting a new save.
I have added all playable English teams to leagues on my llm save. Hope this was helpful.
This is only a basic run down but if you follow it, you should have no problem.