u use an image editor such as GIMP or Photoshop or Paint Shop Pro.
make seperate layers and using sprites or images as u want to use, paste them into those seperate layers. save as .gif and set the time between frames/layers. such as 100ms per layer, as in 100ms, they will switch from one frame/layer to the next. i prefer 80ms, for not many frames.
idealy, if u want a smooth animated gif, such as like my example, make many layers, between 10-20 frames on one move, such as a jump or backflip, and move it minutely, possibly 1-5 pixels, and place at 30-50ms
if you want a fast animated gif, such as something flying through the air, place 5-10 frames and seperate the frames like 20-25 pixels apart from each layer and set the time to 70ms.
experiment, and pm me if u need details.