My description tof a trip to the supermarket

When I need to buy chicken breasts, I do the following: First, Iif I am shirt-less, I put on a shirt.

I take my bike, open the door, and being at the threshold I make sure that I have my bike lock's keys and my mask (here for entering to a supermarket it is necessary to usewear a mask).

"It is necessary" is fine to use, but a more informal way to say it would be "we have to" or "you have to." In general, you can use "you have to" to mean "es necesario" or "se necesita."

After that, I bike to a supermarket that is near my house.

I stop somea few meters before the entrance, at a placwhere wthere is a big and long guardrail lies, and lock my bike to the rail or to one of its posts.

You can say "where a big and long guardrail lies" but it sounds unnecessarily formal or old-fashioned.

I enter to the supermarket, and go directly to the butcher shop.

If there is butcher is already helping someone, I just stand in line after him; if not, I ask the butcher for onfor one or more big chicken breast or mores (depending on the day and thehow much money I gethave).

This butcher shop has its own employees and is, in some sense, separate from the supermarket, but still you shouldhave to pay for your order in the supermarket's cash registerheckout.

"Should" means that it isn't necessary but that it is recommended.

I go to the registers and select the one with lessthe fewest people or lessthe fewest things to be paybought (because sometimes there is just aone person with a big shopplot of things thougho buy).

Alternatives:
... sometimes there is just one person buying a lot of things
... sometimes there is just one person with a full shopping cart

I wait as long as isI needed to and pay for my chicken.

I go out and do the formerprevious actions in reverse: I unlock, get on and ride my bike, ride home, make sure I have the chicken and the change, and finally I throw open the door and take off my shirt.