This is only a description of the justifications for the steps. How you describe the justifications in the proof checker will depend on that tool.
Lines 5 and 6 used universal elimination to replace the variable x with the name a. You will likely need to reference which lines were used.
Line 8 used conditional elimination which should reference the conditional on line 6 and its antecedent on line 7. This gives you the consequent.
Similarly line 9 used conditional elimination which should reference the conditional on line 5 and its antecedent on line 8. This allows you to derive the consequent.
Line 10 derives the contradiction between lines 9 and 4. This is contradiction introduction.
Line 11 derives the negation of the assumption on line 7 discharging the subproof on lines 7 to 10. This is negation introduction.
Line 12 uses existential introduction. This is copied to the last line where existential elimination was used. This existential elimination began with the assumption on line 4 replacing the variable x in line 3 with the name a. This subproof was able to be closed when a line was derived that did not use the name a. That happened on line 12.