One-sided UMP test to Poisson

Question

I have this one r. s. of $X \sim \text{Poisson}(\theta)$ and I need to find the UMP test to $H_0:\theta = \theta_0$ against $H_1:\theta < \theta_0$. And, given $\alpha = 0.05, \, \theta_0 = 1$ and $n = 25$, I need to construct the power function graph (with the theorem of central limit). I more or less understood when $H_1:\theta > \theta_0$, nut now I'm lost! Would it be too much to ask if someone had a step by step on how to solve this? I'm just starting graduation (this is not homework)!