For payslips:
(bb) Payslips must be:
(i) formal payslips issued by the employer and showing the employer’s name; or
(ii) accompanied by a letter from the employer, on the employer’s headed paper and signed by a senior official, confirming the payslips are authentic;
For the employment letter:
(b) A letter from the employer(s) who issued the payslips at paragraph 2(a) confirming:
(i) the person’s employment and gross annual salary;
(ii) the length of their employment;
(iii) the period over which they have been or were paid the level of salary relied upon in the application; and
(iv) the type of employment (permanent, fixed-term contract or agency).
All the details come from here:
https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-fm-se-family-members-specified-evidence