The most important skill you will need to have if you are to start your own firm is how to attract and keep clients needing your services. If you are not an expert at this already, or going to be working with someone who is, then starting your own firm might not be for you.
A common way to start a new law firm is to take your clients from your previous firm with you. Clients are usually loyal and, unless they need other areas of expertise you cannot offer, high levels of PI insurance etc., will be happy to follow you. You should of course check your restrictive covenants, which may prevent this, though enforcement is relatively rare and there is an argument that former firms may be in breach of SRA rules if they stop clients following departing lawyers. If this is your business plan, then you would be wise to pause and also think about the long term. If your client following will continue to produce work for you over many years then you have nothing to consider further, but the chances are the billings from your following will gradually fall. This may be because of pressures on their businesses, stiff competition from other firms for their custom, or their needs change. In this case you will need to supplement your following with new clients won at your new firm. Many firms do well in this respect, but do bear this in mind: you won your client following at your previous firm supported by their investment (marketing, PR, colleagues, websites etc). If that was more substantial than your new firm then you should expect that securing similar clients with your more modest resources may present something of a challenge and this may go some way to explaining why it is not uncommon for new firms to fail in their third year.
You should also bear in mind what your clients need. They usually like a one-stop-shop so a client may not follow where the new firm cannot offer all it needs.
There is an alternative solution if you seek all the flexibility and freedom of setting up your own law firm but wish to avoid the investment inherent in setting up.