The easiest way is to select the outline of your shape and offset it (offset meaning to duplicate the path and set it off at a specified distance). Just go to:
Object > Path > Offset Path
(对象 > 路径 > 偏移路径 piān yí lù jìng)
Then put “3mm” (or whatever) in the first box, which is Offset (位移 wèi yí). You need to leave the other options as the default (mitre), but if you’re curious here’s what they mean:
Joins (连接 lián jiē) is how the offset path treats the corners (anywhere where two lines meet). Miter (斜接 xié jiē) means there is no special treatment, it will replicate your original shape exactly, just bigger or smaller. Round (圆角 yuán jiǎo) is for rounded joins and bevel (斜角 xié jiǎo) keep the length of your paths the same, but draw a line between each join, so that your sharp-angle corners turn into a straight line, like this:
You’re likely reading this because you’ve noticed that the Crop Marks (裁剪标记 cái jiǎn biāo jì) in the Effects menu (效果 xiào guǒ) doesn’t work for irregular shapes.
The good news is that you don’t need crop marks for irregular shapes, just the bleed.
This is because irregular shapes are not cut with a big, straight blade, but die cut (模切 mú qiē). A “die” is a custom shaped bit of metal that fits in a press and cuts out the exact shape of your printed object. Here’s a video of one. As far as I know, if you’re printing large quantities of irregular shaped designs (gift boxes, etc) this is still the process used.
For signs, vinyl, “kiss-cut” stickers, a cutting plotter (刻字机 kè zì jī） is used. This is basically a blade on a robot arm to cut out the shapes. You can get home-use versions now (like this). But for the same reason as die-cut printing, you don’t need crop marks.