Your text: "Hold the backrest adjustment belt with one hand and hold the backrest adjustment clip with the other hand ..."

Unfolding the seat
**Hold the backrest adjustment** device with one hand as shown in Figure 1, then move the backrest
upwards as shown in Figure 2, until the seat is fully opened.



Desplegar el asiento (your text has "adjustment belt" as opposed to "device"). However, IMO, "sujetar" is a far nicer word than "cojer" (https://www.dictionary.com/e/slang/coger/). I am a little surprised that you would even select this word, as it can be construed, depending on context, of course, as vulgar.

https://www.nurse.es/wp-content/uploads/folk-1.pdf (page 15)
Como se muestra en la Figura 1, ***sujete el dispositivo de ajuste del respaldo con una mano,***
luego mueva el respaldo hacia arriba como se muestra en la Figura 2, hasta que el asiento esté
completamente abierto.

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Note added at 27 mins (2021-08-22 13:51:32 GMT)
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Oops ... sorry about the typo. It should be "coger" https://www.urbandictionary.com/define.php?term=Coger

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Note added at 2 hrs (2021-08-22 15:58:40 GMT)
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If you still need help with the second part, "and hold the backrest adjustment clip with the other hand ...", the reason for the non-inclusion of this part of the phrase is that ProZ recommends a limit of 10 words or so, and already you had asked for 8 other words. Eighteen words is too much, so the suggestion is that you split your questions with no more than 10 words or so (11 or perhaps 12 words is probably okay), but ... The reason for this is that when glossary entries are made, very long entries are problematic (too cumbersome).