There are two ways of doing it as has been mentioned in previous threads.
You can claim the whole allowance one time (it was up to a total of 100k) or you can write it down as capital allowances.
Once you have chosen your route you have to stick with it. In other words you can't decide to write the van off as a capital allowance and than change your mind in year 2 and want to write it off as you can't. (You can only write the amount off when the van has reached a tax value of £1000.00 or less.)
If you are new to this, then the chances are that you window cleaning income will still be low, so you need to evaluate where you are with regard to your tax liability. If you have earned a little more than your personal tax allowance (the first amount you aren't taxed on,) then you won't benefit by writing the whole value of the van off.
Capital allowance works on a first year percentage write down and then smaller percentage write-down each year after than on the new pool.
So as an example; you buy a van for £6000.00. These figures do change and need to be verified.
The first year you would be able to claim 40% of that amount against tax; = £2400.00.
Your new 'pool' for the next tax year is £3600.
The capital tax allowance on your van in the second tax year is only 18% of the pool you carried over; £3600 x 18% = £648.00.
Your new 'pool' for the third tax year is £2952.00
The capital allowance for your van for the third tax year is again 18% of the 'pool' you carried from year 2; £2952 x 18% = £532.00
These figures get a little more complicated as very few of us can prove to the receiver that we only use our van for business 100% of the time. If you use the van to pop up the the shops at lunchtime once for a bag of chips, that's private mileage according to the receiver.
So although we have a dedicated wfp van, we claim 90% as business and 10% as private mileage. (We do about 3500 miles per year so we are happy with this. If you do more miles then you may change that percentage. Whilst this is self assessment and you are reporting the way you see your business performing for tax purposes, you also have to justify this to the receiver.)
In our case, we would claim 90% of the £2400 as business (£2160.00) and the remainder will be private use.
On year 2 the amount we would claim against tax would be 90% of £648.00 (£584.00).
Year 3 would would claim 90% of £532.00 (£479.00). In the examples, claiming the business portion of the van value doesn't change the pool value that is carried across to the following year.
In the above example, your van is worth £2420 in year 3. If you sold it for £3000.00 you will have to pay tax on the 'profit' of £580.00. If you sold the van for £2000.00 then you would have a further claim of £420.00 against tax.
It is complicated, so I would suggest either to employ the services of an accountant or phone up the receiver and book yourself onto a course they run occasionally for self employed people. It was free at one time.