In the name of Allah, the most Beneficent, the most Merciful.

Answer

It is obligatory to do istibraa until one is sure that no drop of urine remains in the penis after urinating.

1. The most common method used for istibraa is wrapping tissue around the penis and then returning to the toilet after a few minutes to remove the tissue.
2. Leaning back and forth towards the left side can be used to perform the method of istibraa.
3. Moving up and down a little taking care that no urine falls on the lower garment.
4. Coughing

(Maraqi Falah p.18)

With regards to you question, you should make sure that every drop of urine passes by until your heart is at ease. The essential thing is that one is free from urine before one does wudhu because anything that comes out during or after wudhu invalidates the wudhu.

After that if one fears that baseless misgivings may occur one should sprinkle water on ones undergarments. Then if there is doubt assume that it is from the wetness of the sprinkling. (Raddul Muhtar p.559 v.1)

Only Allah Knows Best

Mohammed Tosir Miah

Darul Ifta Birmingham