Here are some of the common issues with brush lettering beginners:
Go slow and focus when you are practising your upstrokes. Movement should be generated at your wrist when you create a thin upstroke.
Focus on consistency to build your muscle memory. The small brush tips create finer and thinner upstrokes. If you notice you have thick upstrokes, try to add more pressure on your downstrokes to create contrast in a letter.
Use guideline sheets to keep a consistent slant. Hold your pen 45 degrees to the paper to create a slant line. Always reflect and look at the slant of the previous stroke to improve your consistency.
Collapsed loops are due to transitioning too early from thick to thin or vice versa, not enough contrast between down and upstrokes. Always go slow and make a conscious effort to be consistent
5.Heavy/ Uneven Underturn
It is common to see beginners create heavy bottom under turn or uneven pressure on transitioning between thick/ thin strokes. Start off with full pressure downstrokes and when you move toward the bottom/baseline, slowly reduce the wrist pressure to create a thin upstroke