What is the best way to deliver projects on time while handling multiple projects at the same time?

I think basic rule is NOT PANIC that you will not achieve everything on time.
There are ways to prolongate some projects for better.
Better little late and excellent then on time and not successful.
Make daily list what you must do not to forget something which can be important after and at the moment looks like no priority.
Sometimes listen your instinct combine with analyze

think to deliver your project on time belong how much is important to deliver it for your boss to achieve good performance you have to arrange the projects which is urgent or as soon as possible or normal tasks also it's belong to you how can you make your self flexible with all tasks

تقسيم وقتك بالنسبة المئوية وتعيين لكل مشروع نسبة مئوية حسب الأولوية ووفقا للجدولة الزمنية الرئيسية Divide your time by percentage and assign each project a percentage per priority and in accordance with the principale time schedule

I am sorry there is no straight answer for that.
Every project is unique and thus the management of each project is unique as well but I will say “By employing recognized project management practices”.
1- Give enough time in planning, especially in scheduling and estimation.
A project manager can easily avoid conflict between his time and project resources.
2- Plan for risks.
Remember? If anything can go wrong, it will go wrong
3- Advocate for your projects.
Define priorities and manage resources.
4- Manage and control changes.
Silly change requests (especially unofficial) can through you a way off the track.
5- Use earn value analysis extensively.
Gives you over all project standing.
One can prioritize its efforts on the basis of result.

Honestly, multitasking does not exist in the human mind nor in a CPU.
But we find ourselves working in multiple projects and the best approach here is to identify those elements that are likely to cause bottlenecks and maintain a separate tracking of those items at highest priority.

Finde the confused items by making time line fore all project And start repairing for it

Here are some of the techniques to try when managing multiple projects: Time management:  Many times project managers who are overloaded seem to be constantly fighting one crisis after the other.
It just doesn’t seem to stop.
You need to quickly determine what to do when and how much time you have.
You cannot micromanage every single task as a program manager, and you’ll have to start focusing on managing your time among all your projects.
Checklist for managing multiple projects:  You should create a portfolio checklist for managing multiple projects.
You could use a life-cycle checklist to help (e.g., Have I received all the specs? Have I involved QA on each project team?).Prioritization of projects:  You need to determine which projects are more important than the others.
When faced with multiple projects, it’s important to decide what to do and in which order it gets done.
It’s no use simply selecting any project you like and paying attention to that project alone.
Careful analysis is needed to ensure that each project meets your company’s strategic objectives and that you are aware of target dates.
Categorize your work:  This strategy reduces complexity.
You have to make progress on all fronts when managing multiple projects.
Don’t leave one until next week and expect results.
Check the progress of each project on a daily basis.
Sequence work tasks:  Ensure that for every project the tasks have been properly sequenced and that they make sense.
I have seen managers run from project to project, grabbing at straws.
It’s imperative that each project be clearly broken down into manageable tasks and that they be correctly sequenced.
Create a dashboard:  Use a digital dashboard to effectively capture and report on all the projects within your portfolio.
You should also: Determine how much time you have available for each project.
Prioritize your projects according to the client’s business and IT strategies.
Rank all project tasks into A, B, and C categories.
Spend enough time and effort on tasks A for each project, and then proceed to categories B and C as deemed necessary (e.g., technical meetings or workshops).
Notify the client if you cannot meet a deadline or complete a task.
Capture all issues and risks into a centralized project database for efficient reporting.

Use a project tool which allows for managing many projects concurrently and sharing resources such as MS Project Server.

nothing theoretical will be really beneficial to you, and it needs years of experience to do it right...
but as a tip I can advise you with the following: - you need1st to master the following areas: => Scheduling and Planning, preparing realistic schedule is very important, adding buffers, and it should be created by the collaboration of your team, and approved by the management / client.
=> Schedule control, you need to control your schedule by setting as much as possible check points ( weekly, bi-weekly...etc) => Weekly and daily TO DO list, or some people just put every thing in a calender...
i prefer papers ( for mental activity)...
this will prevent u from forgetting ur tasks.
=> Status meetings with your team, every week or bi-weekly or as it needed, but you need to be always aware about the progress.
=> Set with yourself, you need between time to time to pause the daily activities, and to look from above everything...
larger look and quite analyzing usually can help you to figure out where to focus your efforts.

Thank you Shiraz for your valuable question and also thanks to Ferenc for his good detailed answer .
The management of many projects at the same time, is not an easy simple job if the project manager does not has high-level interpersonal skills and good knowledge of all knowledge areas of project management system process to manage such work.
Good collaboration between team members and project managers for all projects in the portfolio of projects of the enterprise project structure in the organization , will help too much in achieving deliverables handing over on time ,under budget and to the specifications and expectations of the stakeholders of the projects.Also good prioritization and categorization of projects facilitate work performance process and tracking of the progress of each project .
The creation of digital dashboard for projects and using efficient management software like Primsvera P6 will be of great help managing and tracking cost,schedule,resources and risk register for all projects.
The organization administration system for project either functional or full project manager authorization will play great exclusive role in managing the projects .and many factors can be mentioned I hope that I have contributed effectively and precisely in answering the question.
Thank you Best regards.

Prioritize your projects, Track the progress of multiple projects with a spreadsheet or another type of software geared toward project management, Schedule resources appropriately between multiple projects, Delegate work to your team members, Check project progress daily, Follow up with all your clients periodically and provide progress reports.