A portrait by any other name or modifier is still a portrait. If I simply add the modifier of MACRO, it doesn't change rules of composition, light, creativity, depth of field, etc. What does change is the way in which the subject is captured. If you use natural light, time of day and angle of the sun still play an important role to determine the success of the photo. But when you make macro portraits, you have a tremendous advantage compared to large mammals or people. Macro portraits can be made in the middle of the day when most nature photographers nap, clean equipment, download, edit, or explore new locations. The advantage is you can easily modify harsh noon-day light with a reflector, diffuser or flash. Depending on the flash's power and dialed-in settings, the sun can be reduced to a source of fill.
To make macro portraits you need to fill the frame with small subjects. While many lenses claim to have macro capabilities, they're not. Some offer macro capability only at the widest point of the zoom which somewhat defeats the purpose of getting close. True macro lenses work wonderfully, but are costly. Depending on the subject matter you most want to photograph, a given focal length will prove more advantageous. For instance, if insects or other skittish animals are your main focus, get a 200mm macro. You can be farther back and not scare off the subject. Additionally, if you use flash, there will be enough working distance where it can evenly light the subject and not have the end of the lens cast a shadow. If you don't want to lay out big bucks for a macro lens, you can use extension tubes, a bellows, teleconverters, or close-up filters. Of the three, I opt for the filters for their ease of use. The key is to use dual element ones made by Nikon and Canon. The Nikon filters are discontinued, but can be purchased on eBay. The single element ones don't give sharp corners. The dual element ones are corrected to prevent this from happening. Extension tubes create distance between the camera body and rear lens element and allow you to focus the lens closer to your subject than it would without them.
Depth of field is critical when shooting macros. It's reduced to millimeters. This makes accurate focusing critical. Make sure the active focus point is precisely over the most important part of the subject. If it's off just a smidgen, the image is destined for the delete button. It helps to use manual focus and rock the camera back and forth to acquire the focus point you desire. To gain as much depth of field as possible, stop the lens down to f/16 or f/22. The trade-off is the corresponding shutter speed may be too slow to handhold or to prevent motion blur. To offset this, shoot when there's a lot of light, bump up your ISO, or use a flash as a main light. Conversely, you may choose to use the lack of depth of field to your advantage and go with selective focus. Open the lens to its widest setting to create a more artistic representation. Explore the subject while you look through the viewfinder to find a composition where a single point of the image is the only sharp part.