You should measure the volume ${V(t)}$ of the oxygen gas $\ce{O2}$ produced by the decomposition reaction vs. time. Do this up to the time when you consider that this volume will not increase any more. This final volume ${V_∞}$ is proportional to the initial $\ce{H2O2}$ concentration, called $c_o$. Now calculate all differences $\Delta V(t) = V_∞ - V(t)$ for all volume measurements. This $\Delta V(t)$ is proportional to the $\ce{H2O2}$ concentration at time $t$.
Then report the successive values of log$\Delta V(t)$ on the $Oy$ axis, and the time $t$ on $Ox$ axis. The points should be alined, and the slope gives you the rate constant $k$. Repeat the same measurements at different temperatures. When these series of measurements are finished, report the log of the different $k$ values vs. ${1/T}$, where $T$ is the absolute temperature in Kelvin. You should obtain a line with a slope equal to $E_a/RT$