I always wait until the cashier has told me what the total is before I put my backpack on the counter and hunt for my checkbook.
Then I spend some time looking for a pen. Sometimes I find one, but it doesn't always work. When that happens, or if I don't find one, I ask the cashier if they have one. (They usually find one after a little while.)
Then I zip up my backpack, sling it over my shoulder and start filling out the check - and the checkbook register. By then I've usually forgotten the amount so I apologize to the cashier (and the next person in line) and ask for the total again. I write it on the check - and in the register. Then I slowly, carefully, gently tear the check out of the checkbook and hand to the cashier. (I also return the pen if it's not mine.)
I wait until the cashier has processed the check and handed me the receipt before I swing my backpack back onto the counter, open it up, put the checkbook in it's pocket, fold up the receipt, put it in *its* pocket and zip the backpack closed again.
Next, I take a peek into each bag to figure out which ones are heavy so I can distribute the weight as I carry them. (Sometimes I move an item or two between bags to even things out.)
Finally, I say "Thank You!" to the cashier, wish him/her a pleasant day, give a nod to the person behind me in line, and pick up my bags.
It's a very smooth and efficient process. I'm sure that the people around me are very impressed.