Sample Code:

global class AccountUndeleteBatchApex implements Database.Batchable<sObject>{
    
    global Database.queryLocator start(Database.BatchableContext bc){
        String query = 'SELECT Id, Name FROM Account WHERE IsDeleted = True ALL ROWS';
        return Database.getQueryLocator(query);
    }
    
    global void execute(Database.BatchableContext bc, List<Account> scope){
        List<Account> accList = new List<Account>();
        for(Account s:scope){
            //Write your logic
            accList.add(s);
        }
        Undelete accList;
    }
    
    global void finish(Database.BatchableContext bc){
        
    }
}