Sit with your back straight, keep your paper comfortably in front of you, and maintain a good posture to reduce hand strain, and you will be able to write smoothly.
Avoid holding the pen too tightly, as it will lead to unnecessary pain in your hand. Choose a pen that is lighter and easier to hold.
Practice with different pens to find the best suited to your hand: ballpoint, gel, etc. Use a smoother point pen. Also, consider lined pages so that you can comfortably write in a straight line.
Writing very quickly leads to sloppy handwriting. Take time to write a sentence so that it will look neat and clean.
Leave appropriate spaces between the letters. Consistent spacing leads to greater readability of your writing.
Try to create uniformity in the sizes of the letters used; pay close attention to the uppercase and the lowercase letters as they make an organised appearance.
Consistency is the key. Spare 10 mins daily to write a paragraph; you can also use a workbook for better results.