Start thinking up match ideas you really like, that haven't been done yet of course. Once you've got that down, think up a scenario to have them fight. It's up to you whether you want them to just throw down with their skills, or go beyond a traditional fight and write a story.
Then write out the fight itself, describing how the combatants' abilities bounce off of each other. You can take inspiration from other fights and pieces of media to add stuff like references to series, but be original enough to make the fight stand out as your own.
Don't worry about being unsure of your skills; I myself was unsure of my skills when I started out. No one starts out perfect. As they say, Rome wasn't built in a day.
Also, if you're starting up a written Death Battle series, I wouldn't recommend using this site anymore. DA recently stopped supporting its sta.sh writer, rendering it unusable to longtime writers. and scaring them off the platform with its downright terrible literature editor. I myself have moved to Wordpress to continue my DB writing, and I suggest you follow me there, though I will be linking my fights from there to here.nathanthemanthemhfan.wordpress…
But if you do want to write here on DA, I recommend just writing on google docs and pasting the link to your deviation.