I'll assume you can get the editor open, and load the database and go from there.
First you'll want to create your new team. Click on Database on the top left menu, and then Clubs down the left side menu.
At the bottom right of the screen are a few buttons, click Add
This will give you a new screen where you'll set all the details for your team.
Use the new menu on the left to navigate through the information.
Details - Club name, city, nation - the basics
Stadium - You can either choose one in the database, or create a new one from scratch, or duplicate one and change the name.
Finances - Set the clubs bank balance, annual transfer fee (for the first season) and weekly wage budgets.
Competition - Set the teams current league (click the grey arrow to the right and then use the search tool to find the English Premier League). The regional Division tab lets you dictate which leagues the team will fall into, it's only relevant for Blue Square North/South and below (in England)
Tactical Attributes - I think these are settings for the AI, but i assume you'll control the team so probably safe to overlook them.
Supporter profile - Does what it says, decide how loyal or patient the fans are
Contract Types - Again, safe to let this pass
Kits and colours - Choose the teams shirts, shorts and socks style and colour. Also set the Circlue icon for 2D pitch view and the colour for the text bar in game.
Staff and Players - Any staff or players you want from the beginning, add them here. Flick through the additional tabs and search using the add button.
Relationship - List any rival teams, club legends or disliked people (they generally wont become manager or team member when computer controlled)
Records - As a new team, you wont need this
So once you made the team you need to sort the leagues out. If you set the competition during team creation, you'll find the game will fail to load as 21 teams will be in the EPL. To move them around, open up the league a team will move into eg. npower Championship, and then add the team to move eg. West Ham. You'll need to repeat this process all the way down to level 9 of the English Pyramid (County Leagues: North West Counties, Spartan South Midlands, Hellenic Leagues etc) as the database charts teams progress this deep even though you can only play to BSB North/South.
The alternative option is to exile a team, search for them on the Club screen (where we went to add your new team), open their record and set their current Competition to None. The team will still exist, but take no part in any competitions, players will likely transfer away and within a few years the team will drop into obscurity.
Save the database, load it up in Fm13 and away you go.