You'll still need your marker and you may want to get some paper of a heavier stock to help with your assignment. If you've printed off and cut out a symbol, you could trace it on heavier paper and get a sturdier stencil (remember sissy: losing or accidentally destroying your art project is unacceptable!).
Like neglecting to repeat your mantra proves, sissies are pathetic and forgetful. So you'll start by picking where you want your tanned-on tattoo to go. Pick a surface you can reach and keep flat while tanning. Now take your marker (a Sharpie works well) and trace your symbol. This is to help remind you to go tanning and to help you know where to put your symbol before the contrast becomes noticeable (unlike your tanned skin, a "permanent" marker will usually wash off in 2-4 days).
Now either tape or - if you're using heavier paper - glue the symbol in place before you start tanning.
Heavier paper also plays a role here. Thinner paper won't block as much UV radiation, so it'll take longer to notice. You can opt to tan outside or at a tanning salon, whichever is easier (a tanning salon will likely be easier to keep private and on time). Working on sissy tanlines with a bra and panties or two-piece swimming suit is optional, but encouraged (most girls like to avoid having lines).
Natalie would be pissed if I didn't make some disclaimer for some of you dumber bimbo sissies: don't tan every day and don't bake yourself in the sun for prolonged periods - you'll risk ruining your tan and that will not make your mistress happy.
And you had better keep your symbol intact and in perfect condition. Too bad if someone might see it or find it - your mistress demands that you keep it safe and that you continue to use it until you have a clearly visible sissy symbol (not)-tanned onto your skin. Feel free to make a duplicate and use that for the actual tanning.
And remember sissy: on days when you have to tan you are not allowed to cum or even touch your pathetic excuse for a clit until after you've tanned. Obviously you shouldn't be touching it at all, but I'm realistic in my expectations of what horny little sissies will do.
I'll make a good sissy out of you yet.