The bottom cover needs to be slid back which is exactly what is done when a tape is loaded by a recorder. There are two tabs that need to be depressed simultaneously to slide the cover back. This also releases the nylon clutches which keep the tape from turning when the tape is not loaded.
Once the bottom cover is moved it locks into position. You have to move the protective (hinged) tape cover out of the way. I use some interdental cleaners for this as in the picture. I also use them to hold the tape in place within the splicing block.
Very importantly the splice should be applied to the backside away from contacting the head when the tape is playing. This doesn't really apply in this example because the splice is at the leader but you will want to do that for sure if the tape breaks somewhere in the middle of the recording.
Cut and apply a section of splicing tape. And then use something to press down on the splice to ensure that there is a good contact. Trim the excess with scissors.
It's entirely possible to make a mistake of course. You might have flipped the tape over the wrong way. If that happens to you, then you won't hear any recording. Redo the procedure and flip it the right way. Hey, it's an opportunity to practice your tape splicing skills!
With a steady hand and a bit of patience, you might just get that prized recording back!