Use the UCAS portal to apply, choosing Durham University (code D86).
Send scanned copies of your transcripts and diplomas, together with your name, date of birth, and UCAS PID, to the university's recruitment and admissions office. 
Make sure your credentials satisfy the particular requirements for the program you have chosen because Durham University evaluates international degrees according to how well they match UK degrees. 
Durham University states that an overall IELTS score of 6.5 is the lowest acceptable score, with no subscore falling below 6.0. Cambridge English and the TOEFL iBT are further recognised assessments. The Durham University International Study Centre offers foundation programs and gateway programs if you don't meet the prerequisites for direct undergraduate study.