Lower arm, inner thigh or scrotal (or labial if post OP). Scrotal has the best absorption, but some can't handle the dysphoria of applying it and it's kinda inconvenient compared to using the arm. Takes about 3-10 minutes to dry depending on dosage vs surface area and also the brand, some contain additives that ease absorption. It's generally easy to fit into my day, i just rub it on after i've showered and apply a second dose in the evening. One thing to keep in mind with estradiole gels is that some of it stays in the skin for a long time and can fuck with blood test results when the blood is taken from the area where the gel is applied, so it's advised not to use the arm you usually get blood drawn from for at least a week (better two to make sure) before the test or your E2 will be tested way higher than it actually is.

I'd switch to injections if these where more readily available around here, blood levels are more stable with them, but i'd either need to import them from the Czech Republic or have them custom made, and if my endocrinologist would prescribe either to me health insurance may rip her head off due to the coast. But injections are much more common in the US AFAIK.