First you need to learn how to sit correctly when writing. At first it will not be easy, but then the correct body position when writing will become a habit. So, sit up straight, keep shoulders back and torso straight, slightly tilt your head forward, lean back on the chair. Do not tilt the torso forward and do not lean breast on the table! Do not place one foot on another, better, bend both knees at a right angle, ensure that the feet touch the floor. Put your hands on the table, leaning on them. Elbows must be behind the edge of the countertop.
Once you learn to sit correctly, also learn to hold a pen. Surprisingly, not all adults properly hold a pen when writing. Someone incorrectly taught to write as a child, and someone eventually retrained myself. In any case, you need to practice a little. The handle should be put on the left side of the middle finger, the index finger holding it from the top and a large bottom. The distance from the index finger to the tip of the handle should be about 1.5-2.5 cm Fingers should be neither too relaxed nor too tense. When writing, the hand should not hang in the air, but to rely on Littlefinger.
Learning to sit correctly and hold a pen, take several samples of writing and practice. Don't try to write entire words and sentences, first, learn how to draw a smooth and beautiful lines, to write individual letters and bundles and only then – the words. Do not attempt to write quickly, speed writing will come with time.
Once you learn how to write beautiful handwriting, slowly and carefully bringing the letters, start to gradually increase the speed of writing. Write dictation, train at least 10-20 minutes each day, then you will certainly achieve great results.