The standard (default) procedure is the normal three-step procedure: written grievance/then meeting/then decision. If the employee has ceased employment then generally by written agreement the modified grievance procedure can be used. This modified grievance procedure has just two steps: written grievance and basis for grievance/then decision.
In a recent case (City of Bradford MBC v Pratt) a cleaner submitted her grievance regarding an equal pay claim and the basis being that she was paid less than male workers whose work was similar or of equal value to hers. She didn’t specify particular comparisons. The case determined that this was not sufficient and therefore she was barred from pursuing her claim. The reason was that there was insufficient detail for the employer to respond.
The amount of detail depends on the nature of the grievance. If there is enough for the employer to find an answer e.g. through investigation / witness statements then it may be sufficient.