The image on the left shows what tools are available if you are in the Edges selection, and the image on the right shows the tools for Vertices. You will find that you often use Remove, Bride, Connect, Chamfer, Target Weld, and Weld.

Some of the tools have a box icon next to them. With these, if you click the box instead of the main button, you will get a window that opens with several options to choose from. You will click the box most often. Clicking the button will use the tool with either the default settings, or whatever settings you entered into the box last.

Go to vertex select mode, and grab the verts of the back of the box in the left viewport and pull them back to the heel.

Go to edge select mode and select the verticle edges (the front and back of the foot) and click the box next to the Connect tool. The window pops up - we only need one segment right now, so the defaults are fine.

In the Top View select the edges along the heel and toe and repeat. Now you've made a division down the center of the side and top of the foot. From the top now select the sides of the foot and click the button next to connect, this time we want 2 segments.

Go back to vertex mode now and start moving things around in the different viewports so it matches the shape of the foot. Switch between the side and top viewports to get the shape better. For the part of the foot where it starts coming up in the front, I select the edges along the top of the foot, but not the bottom and hit connect (only 1 segment) then connected the edge verts to one of the near verts to avoid n-gons.

Go to the Perspective Viewport now and take a look at the foot. Obviously it's not gonna be quite right, but it's closer. Start bringing in areas like the top of the shoe, around the heel, etc.

The temptation will be to start adding more geometry so you can match the shape better, but TRY NOT TO. You will add detail with the texture. Think of this as a big lump of clay and you're just aiming to get the general mass and shape of the object. Try to achieve the best shape you can with as little geometry as possible. There's no point waisting a bunch of verts on the feet when you've got a tight budget.

But Some new geometry is sometimes nessecary... I decided that I wanted to define the heel of the shoe, so I selected the edges circled above and clicked connect with the box. Change segments to 2 and adjust the values of Slide and Pinch so that it's closer to the actual heel. Then select one row of verts and pull them up.

Make sure to connect the new verts to other verts to avoide n-gons. Tris (3-sided polys) are okay, Quads (4-sided polys) are okay, but not 5 or more.

Check all the various angles frequently to judge if you're getting the volume and shape right.