Although a frog and a human may not seem very similar, both humans and frogs need blood and blood cells to carry oxygen to their internal organs. However, there are several differences between frog and human blood, and observing these differences can make for an interesting project. You can observe human blood and then frog blood under the same microscope, but if you have two microscopes, as a lab likely will, then being able to look from one to the other is very helpful. This project is easiest if you purchase prepared slides.
Put the microscopes on a flat, stable surface and turn them on. Adjust the diaphragm over the light source to admit as much light as possible.
Put the slides onto the stage of each microscope, which is the flat platform under the lenses. Place the slide with the human blood on the stage of one microscope and the slide with the frog blood on the stage of the other microscope. Clip the slides into place using the clips attached to the microscopes' stages.
Focus each microscope at 100X. Adjust the lighting if needed. Then increase the power to 400X.
Look at both blood samples. There are several key similarities and differences that you can examine. First, examine the shape of the red blood cells, or erythrocytes. These are the most common cell in blood. Human erythrocytes are very round and regular. Frog erythrocytes form a more elliptical shape. In addition, human erythrocytes lack a nucleus, but frog erythrocytes have nuclei and are capable of dividing. In a frog erythrocyte, you can see a dark spot in the middle of the cell. This is the nucleus.
Look for white blood cells, or leukocytes. These will be far less numerous than erythrocytes. There are several different types, and human and frog leukocytes are similar. On a stained slide, these cells will take the stain differently than erythrocytes and will show up as being darker and a different color from the other cells. They are also often larger than erythrocytes and have a nucleus, which can be seen as a dark region or regions within the cell.
Look for platelets. Again, there will be few of these in comparison to red blood cells. Humans have platelets, which are cell fragments that help blood clot. Frog blood does not have platelets. Platelets will show up as small, dark spots amongst the blood cells.
TL;DR (Too Long; Didn't Read)
If you are not experienced with using a microscope, you may wish to have someone observe you the first time that you do this to ensure that you are using the microscope correctly and do not damage it.
You may wish to create a drawing of your observations.