I have a fulltime work in UK and have an old business which give me some money every few months, I have my national insurance number as a full time worker, but now any money will come from my freelance work will come directly to my bank account, I want to know what steps to do to pay taxes if any for this freelance work and how they will calculate it while I am not sure how much I get per year exactly.
I would talk to your HMRC tax office they do have guidance on this issue here
First, request that you complete a tax return.
On this tax return, you will complete both the employed and self employed sections. This will give you a total income and tax liability.
You will already have paid some tax via PAYE, but you will have to pay additional tax for any other income. For future years there is the option, depending on amount, to collect extra tax through PAYE to cover the other earnings. If it is likely to be the same for the next few years, this may be a better option than paying a lump sum.
The tax return is now mostly online, and not too bad if your affairs are otherwise simple. The hardest part will be keeping a good record of your other earnings. Remember that you have to keep these records for seven years in case HMRC ever want to audit them, and it's a good idea to have a separate account for the income, or some other way of easily identifying it.