The procedure for determining the necessary length is not difficult, but it's a bit involved and shouldn't be hurried.
Please don't estimate or ball-park your spoke length. Many of us do it. Sometimes it works. Often, sadly, it doesn't. Wrong spoke lengths lead to very unhappy wheel builders, which very well might be you. It's not uncommon for a length that's off by just a single millimeter to be too short or too long.
A variety of online spoke calculators are available for free, such as those offered through most major spoke manufacturers, including Sapim and DT Swiss. Without putting my hands on your rim and hub, it's unfortunately not possible for me to determine an accurate length. Please don't send me your rims and hubs.
If you're unsure of the needed length and you'd rather not fuss with the spoke-length calculation, consider a trip to a local bike shop for help. A friendly reminder, that's work for them. As with most work, a compensation request is not unreasonable. On a related note, a previously used spoke may not be a good reference for determining the correct length. Old spokes are often slightly deformed, which sometimes makes an accurate measurement difficult. Never-mind that needing to replace an existing spoke could be a sign that the original spoke, perhaps, was not an ideal length.
Once you've gone through the steps of correctly determining an accurate length, even just a few times, you'll be a spoke-length pro!