Step 3: Adding extra loops
For each phalanx, add a further 2 loops around the initial loop created in the previous step. On the dorsal side of the finger, move the points around to create the oval-esque wrinkle shape that you should see visible on your own finger as you bend and straighten it. On the palmer side of the finger, bring the edges closer together and push the center edge in slightly. In-between each phalanx, add another edge loop and use this to add volume to the palmer side of the finger, creating a fleshier appearance than that of the dorsal side.
For the fingernail, select the relevant 2 faces on the top of the finger and go Edit Mesh > Extrude. Scale the faces in slightly and rework the shape to sync up with the reference. Select the newly created faces and perform another extrusion and lift the new faces up slightly. Add an edge loop to cut around the width of the nail and use the extra detail to add some roundness to the region.
Refining the index finger
Step 4: Creating more fingers
Once we have the first complete finger, select the object and duplicate it by hitting Ctrl+D or going Edit > Duplicate. Translate it over to match the position of the middle finger and go into component mode to edit the vertices to match the reference. Repeat the step to create the ring and pinky fingers also. It's worth studying your own hand as you place the fingers and pay close attention to the arc created by the knuckles as you look at your fist, palm side up.
I also duplicated the original finger for the thumb, only I deleted a few edge loops to allow for only 2 phalanges. Again, simply pushing and pulling the vertices should do the trick to get the thumb sized up. Also note the orientation of the thumb and its position. Again, study your own thumb to inform your modeling decision-making. You may want to use Soft-Selection to speed up the process of moving points. Hit B on the keyboard to activate or de-activate the setting or turn it on in the tool settings for the Move tool. You can also interactively change the falloff of the tool by holding down the B key and left-mouse dragging in the viewport.
Using Soft-Selection to speed up the process of moving points
Step 5: Bringing the fingers together
With all the fingers in place, it's a case of creating the remainder of the hand and bringing it all together. First, select all the fingers and go Mesh > Combine, followed by Edit > Delete By Type > History. To connect the index finger to the middle finger, select an edge from either finger and go Edit Mesh > Extrude. Now go Edit Mesh > Merge Vertex Tool and weld the newly extruded edge to connect the fingers together. I like to make sure this gap between the fingers is present in my hands as it allows for the presence of the 'webby' regions that exist there. Repeat the step to add extra geometry between all the fingers to bring them together and then create more extrudes to attach the index to the thumb.
Adding the 'webby' regions between the fingers
Step 6: Building the hand
Once all the fingers and the thumb have been connected, we can select a row of edges and perform some larger extrudes to start to fill out the hand. I've built up both the dorsal and the palmer side together so I can make sure the flow of the topology between the two sides comes together. The area of most interest will be the fleshy pad of the thumb, and here I decided to cut into the model using the Interactive Split Polygon tool to rework the direction of the flow of edges. Once you have the hand in place, push and pull the points to reduce the rubber glove look. And although I may sound like a broken record at this point, remember to examine your own hand as you go: study the crease lines, the areas that bulge or stretch and implement those features into the model.
Continuing to build out from the fingers to create the palm and the back of the hand