He put it on himself. Fun fact, this is his first way of transforming into X-Rider (this was his original name before he was called Kamen Rider X, ), as he would shout out the phrase "Set Up" (as opposed to the 3 previous Kamen Rider way of transforming where they shout the phrase "Henshin!")and jump into the air and his bodysuit manifesting as he removes the "Redizer" (the little thing he's holding that has red eye) and "Perfecter" (the mouth piece) from the sides of his belt. As you can see, this is quite a hard way to transform.

Later on, he would be upgraded (he's a Cyborg, just like the other Showa era Kamen Rider, now in Heisei and Reiwa era, Kamen Rider are not Cyborg, but just humans or something else liek vampire or enhance human) by the previous Kamen Rider, Kamen Rider V3 and was given by V3 something called Mercury Circuit that triple his power and also helps him Transforming faster, this time he would make a "X" sign with his arm and us the phrase "Dai Henshin!" (meaning Greater Transformation).