The most straightforward way is to claim the simplified expenses for working from home as detailed here Simplified expenses if you're self-employed: Working from home - GOV.UK
which you can use as long as you work more than 25 hours per month from home. You do have to work out how many hours you work from home each month, and make sure to claim the correct amount each month, you can’t just apply it across the whole year.
You can work out a proportion of your costs but this is far more complicated.
In addition, you can claim a proportion of your phone bills, based on the percentage you use your phone for business.
To claim, simply enter a journal which would debit “Use of home” and credit “Proprietor Drawings”. For telephone, the debit would obviously be to Telephone costs instead.